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a additional note on the question: you must have made an error because using the rewrite rule g[x_] := If[x>=0,True,False] and inputting g[2 y^2] returns If[2 y >= 0, True, False] it does not return False (you probably got a false with a rewrite rule you forgot to clear). the reason is that since y is undefined mathematica can't decide if it is true or false and so it returns the input unevaluated. the rest of my discussion is, i believe, okay. richard j. gaylord, university of illinois, gaylord at ux1.cso.uiuc.edu "if you're not programming functionally, then you must be programming dysfunctionally"