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I'm making a sort of symbolic derivator, which needs to take its formulas in unevaluated form.

SetAttributes[derDer, HoldAll]
...
derDer[Plus[a_, b_], aux_] := Plus[ derDer[a, aux] , derDer[b, aux] ]
derDer[Times[a_, b_], aux_] := Plus[ Times[a, derDer[b, aux]], Times[derDer[a, aux] , b] ]
derDer[a_, aux_] := Condition[derDer[Evaluate[a], aux], Hold[a] != Hold[Evaluate[a]]]

The last line doesn't work well. I want the expression to see if its first (held) argument is equal to its own evaluation, and if it isn't so, run itself again with the evaluated argument. I don't understand how Hold , Eval and HoldAll work well enough to do this.

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I've solved the problem by replacing the last expression with

derDer[a_, aux_] := Condition[derDer[Evaluate[a], aux], Not[Hold[a] === Hold[Evaluate[a]]]]

The problem was == returning unevaluated cases.

Related question.

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