FLAME RETARDANT COMPOSITION AND METHOD BACKGROUND OF THE INVENTION In the plastics industry, many product applications require flame retardant plastic compositions. In some cases, this can be achieved by using inherently flame-retardant plastics, such as halogenated polymers. In other cases, plastics that are not inherently flame-retardant are required, and flame retardant additives must be added to the plastics composition. However, many of the most effective flame retardant additives are halogenated compounds that are currently disfavored for health or environmental reasons. Furthermore, when non-halogenated flame retardant additives are used, they often must be employed in high concentrations to achieve the desired flame retardancy, and these high concentrations detract from the desired physical properties of the plastic composition. There is therefore a need for flame-retardant compositions that are both halogen-free and effective at low concentrations. BRIEF DESCRIPTION OF THE INVENTION The above-described and other drawbacks are alleviated by a flame retardant composition, comprising: a phosphorus salt having the formula (Formula Removed) wherein Md+ is a metal ion or an onium ion; d is 1,2, 3, or 4 according to the identity of M and its oxidation state; each occurrence of Rl and R2 is independently C1-C18 hydrocarbyl; and each occurrence of m and n is independently 0 or 1; and a phosphine compound selected from trihydrocarbylphosphines, trihydrocarbylphosphine oxides, and combinations thereof. Another embodiment is flame-retardant plastic composition, comprising: (a) a thermoplastic resin or a thermoset resin; and (b) a flame retardant comprising (bl) a phosphorus salt having the formula (Formula Removed) wherein Md+ is a metal ion or an onium ion; d is 1, 2, 3, or 4 according to the identity of M and its oxidation state; each occurrence of R1 and R2 is independently C1-C18 hydrocarbyl; and each occurrence of m and n is independently 0 or 1; and (b2) a phosphine compound selected from trihydrocarbylphosphines, trihydrocarbylphosphine oxides, and combinations thereof. Another embodiment is a curable composition, comprising: (a) a functionalized poly(arylene ether) resin; (b) a curable compound selected from triallyl cyanurate, triallyl isocyanurate, epoxy resins, bismaleimide resins, bismaleimide triazine resins, and combinations thereof; and (c) a flame retardant, comprising (cl) a phosphorus salt having the formula (Formula Removed) wherein Md+ is a metal ion or an onium ion; d is 1, 2, 3, or 4 according to the identity of M and its oxidation state; each occurrence of R1 and R2 is independently C1-C18 hydrocarbyl; and each occurrence of m and n is independently 0 or 1; and (C2) a phosphine compound selected from trihydrocarbylphosphines, trihydrocarbylphosphine oxides, and combinations thereof. Other embodiments, including methods of preparing the compositions, articles prepared from the flame-retardant plastic composition, and cured compositions and articles prepared from the curable composition, are described in detail below. DETAILED DESCRIPTION OF THE INVENTION A first category of embodiments relates to the flame retardant composition itself. Thus, one embodiment is a flame retardant composition, comprising: a phosphorus salt having the formula (Formula Removed) wherein Md+ is a metal ion or an onium ion; d is 1, 2, 3, or 4 according to the identity of M and its oxidation state; each occurrence of R1 and R2 is independently C1-C18 hydrocarbyl; and each occurrence of m and n is independently 0 or 1; and a phosphine compound selected from trihydrocarbylphosphines, trihydrocarbylphosphine oxides, and combinations thereof. The present inventors have discovered that the combination of the phosphorus salt and the phosphine compound has a synergistic flame retardant effect that provides improved flame retardancy compared to the individual components. This advantage can be used to reduce the total amount of flame retardant required, thereby improving physical properties of a plastic composition. Alternatively, the advantage can be used to achieve greater flame retardancy (e.g., a UL 94 rating of V-0) than was previously attainable at any tolerable level of flame retardant compound. The flame retardant combination is suitable for use with a wide variety of plastic compositions, including those comprising thermoplastic resins and those comprising thermoset resins. One specific use of the flame retardant composition is as an additive to a curable composition comprising a functionalized poly(arylene ether), and a curable compound such as triallyl cyanurate, triallyl isocyanurate, an epoxy resin, a bismaleimide resin, a bismaleimide triazine resin, or the like. The phosphorus salt used in the flame retardant composition has the formula (Formula Removed) wherein Md+ is a metal ion or an onium ion; d is 1, 2, 3, or 4 according to the identity of M and its oxidation state; each occurrence of R1 and R2 is independently C1-C18 hydrocarbyl; and each occurrence of m and n is independently 0 or 1. As used herein, the term "hydrocarbyl", whether used by itself, or as a prefix, suffix, or fragment of another term, refers to a residue that contains only carbon and hydrogen. The residue may be aliphatic or aromatic, straight-chain, cyclic, bicyclic, branched, saturated, or unsarurated. It may also contain combinations of aliphatic, aromatic, straight chain, cyclic, bicyclic, branched, saturated, and unsaturated hydrocarbon moieties. The hydrocarbyl residue, when so stated however, may contain heteroatoms over and above the carbon and hydrogen members of the substituent residue. Thus, when specifically noted as containing such heteroatoms, the hydrocarbyl or hydrocarbylene residue may also contain carbonyl groups, amino groups, hydroxyl groups, or the like, or it may contain heteroatoms within the backbone of the hydrocarbyl residue. In one embodiment, Md+ is an onium ion. Suitable onium ions include, for example, ammonium cation (NH^, mono-(C1-C12)-hydrocarbylammonium cations, di-(C1- C12)-hydrocarbylammonium cations, tri-(C1-C12)-hydrocarbylammonium cations, tetra-(C1-C12)-hydrocarbylammonium cations, phosphonium cation (PH4+), mono-(C1- C12)-hydrocarbylphosphonium cations, di-(C1-C12)-hydrocarbylphosphonium cations, tri-(C1-C12)-hydrocarbylphosphonium cations, tetra-(C1-C12)- hydrocarbylphosphonium cations, sulfonium cation (SH3+), mono-C1-C12- hydrocarbylsulfonium cations, di-(C1-C12)-hydrocarbyl sulfonium cations, tri-(C1-C12)-hydrocarbyl sulfonium cations, and the like, and combinations thereof. In another embodiment, M + is a metal ion. Suitable metal ions include, for example, ions of magnesium, calcium, aluminum, antimony, tin, germanium, titanium, zinc, iron, zirconium, cerium, bismuth, strontium, manganese, lithium, sodium, potassium, and the like, and combinations thereof. In one embodiment, Md+ is A13+. Referring again to the phosphorus salt structure above, in one embodiment each occurrence of R1 and R2 is independently C1-C6 alkyl. In another embodiment, each occurrence of R1 and R2 is methyl or ethyl. In a preferred embodiment, M is aluminum and each occurrence of m and n is zero. In another preferred embodiment, the phosphorus salt comprises aluminum tris(diethylphosphinate). The flame retardant composition may comprise about 5 to about 95 parts by weight of the phosphorus salt, based on 100 parts by weight total of the phosphorus salt and the phosphine compound. Within this range, the phosphorus salt amount may be at least about 10 parts by weight, or at least about 20 parts by weight. Also within this range, the phosphorus salt amount may be up to about 90 weight percent, or up to about 80 weight percent. In addition to the phosphorus salt, the flame retardant composition comprises a phosphine compound selected from trihydrocarbylphosphines, trihydrocarbylphosphine oxides, and combinations thereof. The phosphine compound may be a trihydrocarbylphosphine. The trihydrocarbylphosphine may have the structure (Structure Removed) wherein R3-R5 are each independently C1-C12 hydrocarbyl, with the proviso that the trihydrocarbylphosphine has at least six carbon atoms. In the context of the trihydrocarbylphosphine and the trihydrocarbylphosphine oxide discussed below, the hydrocarbyl substituent may include, in addition to carbon and hydrogen, a hydroxy substituent (e.g., the hydrocarbyl substituent may be 4-hydroxyphenyl), or an ether oxygen (e.g., the hydrocarbyl substituent may be 4-phenoxyphenyl). Suitable trihydrocarbylphosphines include, for example, triphenylphosphine, allyldiphenylphosphine, diallylphenylphosphine, triallylphosphine, bis(l-naphthyl)(4- hydroxyphenyl)phosphine, bis(4-hydroxyphenyl)(l -naphthyl)phosphine, tris(4- hydroxyphenyl)phosphine, tris(l-naphthyl)phosphine, tris(2-naphthyl)phosphine, bis(4-phenoxyphenyl)(4-hydroxyphenyl)phosphine, bis(4-hydroxyphenyl)(4- phenoxyphenyl)phosphine, tris(4-phenoxyphenyl)phosphine, bis(2,4,5- trimethylphenyl)(4-hydroxyphenyl)phosphine, bis(4-hydroxyphenyl)(2,4,5- trimethylphenyl)phosphine, tris(2,4,5-trimethylphenyl)phosphine, bis(tert-butyl)(4-hydroxyphenyl)phosphine, bis(4-hydroxy-phenyl)(tert-butyl)phosphine, tris(tert-butyl)phosphine, and the like, and combinations thereof. The phosphine compound may be a trihydrocarbylphosphine oxide. The trihydrocarbylphosphine oxide may have the structure (Structure Removed) wherein R3-R5 are each independently C1-C12 hydrocarbyl, with the proviso that the trihydrocarbylphosphine oxide has at least six carbon atoms. Suitable trihydrocarbylphosphine oxides include, for example, triphenylphosphine oxide, allyldiphenylphosphine oxide, diallylphenylphosphine oxide, triallylphosphine oxide, bi s( 1 -naphthyl)(4-hydroxyphenyl)phosphine oxide, bi s(4-hydroxyphenyl)( 1 - naphthyl)phosphine oxide, tris(4-hydroxyphenyl)phosphine oxide, tris(l- naphthyl)phosphine oxide, tris(2-naphthyl)phosphine oxide, bis(4-phenoxyphenyl)(4- hydroxyphenyl)phosphine oxide, bis(4-hydroxyphenyl)(4-phenoxyphenyl)phosphine oxide, tris(4-phenoxyphenyl)phosphine oxide, bis(2,4,5-trimethylphenyl)(4- hydroxyphenyl)phosphine oxide, bis(4-hydroxyphenyl)(2,4,5- trimethylphenyl)phosphine oxide, tris(2,4,5-trimethylphenyl)phosphine oxide, bis(tert-butyl)(4-hydroxyphenyl)phosphine oxide, bis(4-hydroxy-phenyl)(tert- butyl)phosphine oxide, tris(tert-butyl)phosphine oxide, and the like, and combinations thereof. The flame retardant composition may comprise about 5 to about 95 parts by weight of the phosphine compound, based on 100 parts by weight total of the phosphorus salt and the phosphine compound. Within this range, the phosphine compound amount may be at least about 10 parts by weight, or at least about 20 parts by weight. Also within this range, the phosphine compound amount may be up to about 90 weight percent, or up to about 80 weight percent. One embodiment is a flame retardant composition comprising a phosphorus salt having the formula (Formula Removed) wherein Md+ is A13+, each occurrence of R1 and R2 is independently C1-C6hydrocarbyl, and each occurrence of m and n is 0; and a trihydrocarbylphosphine oxide having the structure (Structurer Removed) wherein R3-R5 are each independently C3-C12 hydrocarbyl. One embodiment is a flame retardant composition comprising aluminum tris(diethylphosphinate) and a phosphine oxide selected from triphenylphosphine oxide, allyldiphenylphosphine oxide, and combinations thereof. In one embodiment, the flame retardant composition may be prepared by blending the phosphorus salt and the phosphine compound. However, it is not necessary for these two components to be pre-blended before addition to a polymer composition. For example, as demonstrated in the working examples below, the advantages of the flame retardant combination may be attained if the phosphorus salt and the phosphine compound are added as separate components to a polymer composition that is subsequently intimately blended. The flame retardant composition is useful to impart flame retardancy to a variety of polymeric compositions. Thus, a second category of embodiments relates to a composition, comprising: (a) a thermoplastic resin or a thermoset resin; and (b) a flame retardant comprising (bl) a phosphorus salt having the formula (Formula Removed) wherein M4"1" is a metal ion or an onium ion; d is 1, 2, 3, or 4 according to the identity of M and its oxidation state; each occurrence of R1 and R2 is independently C1-C18 hydrocarbyl; and each occurrence of m and n is independently 0 or 1, and (b2) a phosphine compound selected from trihydrocarbylphosphines, trihydrocarbylphosphine oxides, and combinations thereof. Combinations (blends) of thermoplastic resin and thermoset resin may be used. Thermoplastic resins suitable for use in the composition include, for example, poly(arylene ether)s, poly(arylene sulfide)s, polyamides, polystyrenes including homopolystyrene and rubber-modified polystyrene ("high impact polystyrene" or "HIPS"), polyolefins including polyethylene and polypropylene, polyesters including polyarylates. polycarbonates, poly(styrene-co-acrylonitrile)s ("SAN"), poly(acrylonitrile-co-butadiene-co-styrene)s ("ABS"), poly(styrene-co-maleic anhydride)s ("SMA"), poly(acrylonitrile-costyrene-co-acrylate)s ("ASA"), polyimides, polyamideimides, polyetherimides, polysulfones, polyethersulfones, polyketones, polyetherketones, polysiloxanes, and the like, and combinations thereof. These thermoplastic resins and methods for their preparation are known in the art. Combinations (blends) of the aforementioned thermoplastic resins include, for example, poly(arylene ether)-polyamide blends, poly(arylene ether)-polystyrene blends, poly(arylene ether)-polyolefin blends, polycarbonate-polyester blends, polycarbonate-ABS blends, polycarbonate-polysiloxane blends, and polyetherimide-polysiloxane blends. In one embodiment, the thermoplastic resin comprises a poly(arylene ether). Preferred poly(arylene ether)s include homopolymers of 2,6-dimethylphenol (i.e., poly(2,6-dimethyl-l,4-phenylene ether) and copolymers of 2,6-dimethylphenol and 2,3,6-trimethylphenol (i.e., poly(2,6-dimethyl-l,4-phenylene ether-co-2,3,6-trimethyl-l ,4-phenylene ether)). Thermoset resins suitable for use in the composition include, for example, epoxy resins, unsaturated polyester resins, polyimide resins, bismaleimide resins, bismaleimide triazine resins, cyanate ester resins, vinyl resins, benzoxazine resins, benzocyclobutene resins, acrylics, alkyds, phenol-formaldehyde resins, novolacs, resoles, melamine-formaldehyde resins, urea-formaldehyde resins, hydroxymethylfurans, isocyanates, diallyl phthalate, triallyl cyanurate, triallyl isocyanurate, unsaturated polyesterimides, and the like, and combinations thereof. In one embodiment, the thermoset resin comprises an epoxy resin. In another embodiment, the thermoset resin comprises triallyl cyanurate. In another embodiment, the thermoset resin comprises triallyl isocyanurate. Particularly suitable epoxy resins include those described by the structure (Structure Removed) wherein A is an organic or inorganic radical of valence n, X is oxygen or nitrogen, m is 1 or 2 and consistent with the valence of X, and n is from 1-1000 ideally 2-8, most preferably 2-4. Suitable epoxy resins include those produced by the reaction of epichlorohydrin or epibromohydrin with a phenolic compound. Suitable phenolic compounds include, for example, resorcinol, catechol, hydroquinone, 2,6-dihydroxy naphthalene, 2,7- dihydroxynapthalene, 2-(diphenylphosphoryl)hydroquinone, bis(2,6-dimethylphenol) 2,2''-biphenol, 4,4-biphenol, 2,2'',6,6''-tetramethylbiphenol, 2,2'',3,3'',6,6''- hexamethylbiphenol, 3,3'',5,5''-tetrabromo2,2''6,6''-tetramethylbiphenol, 3,3''- dibromo-2,2>,6,6''-tetramethylbiphenol, 2,2'',6,6''-tetramethyl-3,3''5-dibromobiphenol, 4,4''-isopropylidenediphenol (bisphenol A), 4,4''-isopropylidenebis(2,6- dibromophenol) (tetrabromobisphenol A), 4,4''-isopropylidenebis(2,6-dimethylphenol) (teramethylbisphenol A), 4,4''-isopropylidenebis(2-methylphenol), 4,4''- isopropylidenebis(2-allylphenol), 4,4''(l,3-phenylenediisopropylidene)bisphenol (bisphenol M), 4,4''-isopropylidenebis(3-phenylphenol) 4,4''-(l,4- phenylenediisoproylidene)bisphenol (bisphenol P), 4,4''-ethylidenediphenol (bisphenol E), 4,4''oxydiphenol, 4,4''thiodiphenol, 4,4''thiobis(2,6-dimethylphenol), 4,4''- sufonyldiphenol, 4,4''-sufonylbis(2,6-dimethylphenol) 4,4''sulfinyldiphenol, 4,4''- hexafluoroisoproylidene)bisphenol (Bisphenol AF), 4,4''(1- phenylethylidene)bisphenol (Bisphenol AP), bis(4-hydroxyphenyl)-2,2- dichloroethylene (Bisphenol C), bis(4-hydroxyphenyl)methane (Bisphenol-F), bis(2,6- dimethyl-4-hydroxyphenyl)methane, 4,4''-(cyclopentylidene)diphenol, 4,4''- (cyclohexylidene)diphenol (Bisphenol Z), 4,4''-(cyclododecylidene)diphenol 4,4''-(bicyclo[2.2.1]heptylidene)diphenol, 4,4''-(9H-fluorene-9,9-diyl)diphenol, 3,3-bis(4-hydroxyphenyl)isobenzofuran-l(3H)-one, l-(4-hydroxyphenyl)-3,3-dimethyl-2,3-dihydro-lH-inden-5-ol, l-(4-hydroxy-3,5-dimethylphenyl)-l,3,3,4,6-pentamethyl-2,3-dihydro-lH-inden-5-ol, 3,3,3'',3''-tetramethyl-2,2l,3)3''-tetrahydro-l,r-spirobi[indene]-5,6''-diol (Spirobiindane), dihydroxybenzophenone (bisphenol K), tris(4-hydroxyphenyl)methane, tris(4-hydroxyphenyl)ethane, tris(4-hydroxyphenyl)propane, tris(4-hydroxyphenyl)butane, tris(3-methyl-4-hydroxyphenyl)methane, tris(3,5-dimethyl-4-hydroxyf)henyl)methane, tetrakis(4-hydroxyphenyl)ethane, tetrakis(3,5- dimethyl-4-hydroxyphenyl)ethane, bis(4-hydroxyphenyl)phenylphosphine oxide, dicyclopentadienylbis(2,6-dimethyl phenol), dicyclopentadienyl bis(2-methylphenol), dicyclopentadienyl bisphenol, and the like, and mixtures thereof. Other suitable epoxy resins include N-glycidyl phthalimide, N-glycidyl tetrahydrophthalimide, phenyl glycidyl ether, p-butylphenyl glycidyl ether, styrene oxide, neohexene oxide, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, tetramethyleneglycol diglycidyl ether, polytetramethylene glycol diglycidyl ether, bisphenol A-type epoxy compounds, bisphenol S-type epoxy compounds, resorcinol-type epoxy compounds, phenol novolac-type epoxy compounds, cresol novolac-type epoxy compounds, adipic acid diglycidyl ester, sebacic acid diglycidyl ester, phthalic acid diglycidyl ester, and the like, and mixtures thereof. Also suitable as epoxy resins are the glycidyl ethers of phenolic resins such as the glycidyl ethers of phenol-formaldehyde novolac, alkyl substituted phenol- formaldehyde resins including cresol-formaldehyde novolac, /-butylphenol-formaldehyde novolac, sec-burylphenol- formaldehyde novolac, tert-octylphenol-formaldehyde novolac, cumylphenol- formaldehyde novolac, decylphenol-formaldehdye novolacs, and the like. Other useful epoxies are the glycidyl ethers of bromophenol-formaldehdye novolac, chlorophenol-formaldehyde novolac, phenol-bis(hydroxymethyl)benzene novolac, phenol-bis(hydroxymethylbiphenyl) novolac, phenol-hydroxybenzaldehyde novolac, phenol-dicyclopentadiene novolac, naphthol-formaldehyde novolac, naphthol- bis(hydroxymethyl)benzene novolac, naphthol-bis(hydroxyrnethylbiphenyl) novolac, naphthol-hydroxybenzaldehyde novolac, naphthol-dicyclopentadiene novolacs, and the like, and mixtures thereof. Also suitable as epoxy resins are the polyglycidyl ethers of polyhydric aliphatic alcohols. Examples of such polyhydric alcohols that may be mentioned are 1,4-butanediol, 1,6-hexanediol, polyalkylene glycols, glycerol, trimethylolpropane, 2,2-bis(4-hydroxy-cyclohexyl)propane and pentaerythritol. Curing agents for the epoxy resins include amine compounds, anhydrides, benzenediol compounds, bisphenol resin, polyhydric phenol resin, phenolic resins, and the like. Examples of the amine compounds include aliphatic amine compounds, such as diethylene triamine (DETA), triethylene tetramine (TETA), tetraethylene pentamine (TEPA), diethylaminopropylamine (DEAPA), methylene diamine, N- aminoethylpyrazine (AEP), m-xylylene diamine (MXDA) and the like; aromatic amine compounds such as m-phenylene diamine (MPDA), 4,4''- diaminodiphenylmethane (MDA), diaminodiphenylsulfone (DADPS), diaminodiphenyl ether and the like; and secondary or tertiary amine compounds such as phenylmethyldimethylamine (BDMA), dimethylaminomethylphenol (DMP-10), tris(dimethylaminomethyl)phenol (DMP-30), piperidine, 4,4''- diaminodicyclohexylmethane, 1,4-diaminocyclohexane, 2,6-diaminopyridine, m-phenylenediamine, p-phenylenediamine, 4,4''-diaminodiphenylmethane, 2,2''-bis(4-aminophenyl)propane, benzidine, 4,4''-diaminophenyl oxide, 4,4''-diaminodiphenylsulfone, bis(4-aminophenyl)phenylphosphine oxide, bis(4-aminophenyl)methylamine, 1,5-diaminonaphthalene, m-xylenediamine, p-xylenediamine, hexamethylenediamime, 6,6''-diamine-2,2''-pyridyl, 4,4''-diaminobenzophenone, 4,4''-diaminoazobenzene, bis(4-aminophenyl)phenylmethane, 1,1 -bis(4-aminophenyl)cyclohexane, 1,1 -bis(4-amino-3-methylphenyl)cyclohexane, 2,5-bis(m-aminophenyl)-l,3,4-oxadiazole, 2,5-bis(p-aminophenyl)-l,3,4-oxadiazole, 2,5-bis(m-aminophenyl)thiazo(4,5-d)thiazole, 5,5''-di(m-aminophenyl)-(2,2'')-bis-(1,3,4-oxadiazolyl), 4,4''-diaminodiphenylether, 4,4''-bis(p-aminophenyl)-2,2''-dithiazole, m-bis(4-p-aminophenyl-2-thiazolyl)benzene, 4,4''-diaminobenzanilide, 4,4''-diatninophenyl benzoate, N,N''-bis(4-aminobenzyl)-p-phenylenediamine, and 4,4''-methylenebis(2-chloroaniline); melamine, 2-amino-s-triazine, 2-amino-4-phenyl-s-triazine, 2-amino-4-phenyl-s-triazine, 2-amino-4,6-diethyl-s-triazine, 2-atnino-4,6-diphenyl-s-triazine, 2-amino-4,6-bis(p-methoxyphenyl)-s-triazine, 2-arnino-4-anilino-s-triazine, 2-amino-4-phenoxy-s-triazine, 2-amino-4-chloro-s-triazine, 2-amino-4-aminomethyl-6-chloro-s-triazine, 2-(p-aminophenyl)-4,6-dichloro-s-triazine, 2,4-diamino-s-triazine, 2,4-diamino-6-methyl-s-triazine, 2,4-diamino-6-phenyl-s-triazine, 2,4-diamino-6-benzyl-s-triazine, 2,4-diammo-6-(p-aminophenyl)-s-triazine, 2,4- diammo-6-(m-aminophenyl)-s-triazine, 4-amino-6-phenyl-s-triazine-2-ol, and 6-amino-s-triazine-2,4-diol, and the like, and mixtures thereof. Suitable cyanate ester resins include compounds of structure (Structure Removed) wherein A is an organic or inorganic radical of valence n; and n is from 1-1000 ideally 2-8, most preferably 2-4. Suitable cyanate esters useful include cyanatobenzene, 1,3- 4-cumylcyanatobenzene, dicyanatobenzene, 2-f-butylcyanatobenzene, 2,5-di-f-butyl- 1,4-dicyanatobenzene, 2,5-di-t-butyl-l ,3-dicyanatobenzene, 4-chloro-l ,3- dicyanatobenzene, 1,3,5-tricyanatobenzene, 4,4''-cyanatobiphenyl 2,2''- dicyanatobiphenyl, 2,4-dimethyl-l ,3-dicyanatobenzene, tetramethyldicyanatobenzene, 1,3- dicyanatonaphthalene, 1, 4- dicyanatonaphthalene, 1,5- dicyanatonaphthalene, 1,6- dicyanatonaphthalene, 1,8- dicyanatonaphthalene, 2,6-dicyanatonaphthalene, 2,7- dicyanatonaphthalene, 2,2-bis(3,5-dibromo-4-cyanatophenyl)propane 1,3,6- tricyanatonapthalene, 2,2-bis(4-cyanatophenyl)propane, bis(4- cyanatophenyl)methane, bis(3-chloro-4-cyanatophenyl)methane bis(3,5-dimethyl-4- cyanatophenyl)methane, l,3-bis[4-cyanatophenyl-l-(l-methylethylidene)]benzene, 1,1,1 -tris(4- cyanatophenyl)ethane, 1,4-bis[4-cyanatophenyl-1 -(1 -methylethylidene)]- benzene, and the like, and mixtures thereof. The cyanate ester may be a cyanate ester prepolymer, such as, for example, prepolymers of 2,2-bis(4-cyanatophenyl)-propane, bis(3,5-dimethyl-4-cyanatophenyl)methane, 1,3-bis[4-cyanatophenyl-1 -(1 - methyl ethylidene)]benzene, 1,4-bis[4-cyanatophenyl-1 -(1 -methyl ethylidene)]benzene, bis(4-cyanatophenyl)ether, bis(p- cyanophenoxyphenoxy)benzene, di(4-cyanatophenyl)ketone, bis(4- cyanatophenyl)thioether, bis(4-cyanatophenyl)sulfone, tris(4- cyanatopheriyl)phosphite, and tris(4-cyanatophenyl)phosphate. Also useful are other cyanates as disclosed in U.S. Pat. No. 5,215,860, col. 10, lines 19 to 38. Cyanate ester prepolymers that can be used in the present invention contain free cyanate ester groups and may be produced by partial curing of the cyanate ester resin in the presence or absence of a catalyst. A typical example of such a cyanate ester prepolymer is the partial reaction product of bis(3,5-dimethyl-4-cyanatophenyl)methane, sold under the tradename AroCy® B-30, B-50 M-20, PT-60, PT-60S, and CT-90 by Lonza. Ltd., Switzerland. Mixtures of two or more different cyanate ester prepolymers may be used, as can mixtures of one or more cyanate ester prepolymers with one or more cyanate ester-containing compounds that are not prepolymers. Useful cyanate esters include materials commercially produced by Lonza Ltd., Switzerland and include, for example, B-10, B-30, M-10, M-30, PT-15, PT-30, PT-30S, PT-60, PT-60S, CT-90, BA-230S, L-10, F-10, RTX-399, RTX-366, and Quatrex-7187 resins Metal salt catalysts, such as metal carboxylates can be used to accelerate the cure rate of cyanate esters. Catalysts include manganese naphthenate, zinc naphthenate, cobalt naphthenate, nickel naphthenate, cerium naphthenate, manganese octanoate, zinc octanoate, cobalt octanoate, nickel octanoate and cerium octanoate, and the like. Suitable bismaleimides include those of structure (Sturecture Removed) wherein in M is a radical containing 2-40 carbon atoms of valence n and each Z is independently a hydrogen, halogen or an aromatic or aliphatic radical and n equals 0-10. M can be aliphatic, cycloaliphatic, aromatic or heterocyclic. A preferred class of bisimides is difunctional bismaleimides derived from aliphatic or aromatic diamines. Specific examples of unsaturated imides include 1,2-bismaleimidoethane, 1,6-bismaleimidohexane, 1,3-bismaleimidobenzene, 1,4-bismaleimidobenzene, 2,4-bismaleimidotoluene, 4,4''-bismaleimidodiphenylmethane, 4,4''-bismaleimidodiphenylether, 3,3''-bismaleimidodiphenylsulfone, 4,4''. bismaleimidodiphenylsulfone, 4,4''-bismaleimidodicyclohexylmethane, 3,5-bis(4- maleimidophenyl)pyridine, 2,6-bismaleimidopyridine, 1,3- bis(maleimidomethyl)cyclohexane, 1,3 -bis(maleimidomethyl)benzene, 1,1 -bis(4- maleimidophenyl)cyclohexane, 1,3-bis(dichloromaleimido)benzene, 4,4''- biscitraconimidodiphenylmethane, 2,2-bis(4-maleimidophenyl)propane, l-phenyl-1,1- bis(4-maleimidophenyl)ethane,