| http://www.w3.org/ns/prov#value | - re x2 is 0 to 5 and x3 is 0 to 5, provided that at least one of x2 and x3 is other than 0; [0007] X1 is CH or N; [0008] X2 is C, N, O or S; [0009] X3 is C or N; [0010] X4 is C, N, O or S, provided that at least one of X2, X3 and X4 is N. [0011] In each of X1 through X4, as defined above, C may include CH. [0012] R1 is H or alkyl; [0013] R2 is H, alkyl, alkoxy, halogen, amino or substituted amino;
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