Several issues remain to be addressed:

• Efficiency: how to organize patterns for efficient search.

• Recursion to transform substructures as well as the main expression.

• Repetition to transform an expression as many times as possible.

• Termination: can the program get into a loop by transforming an expression and then transforming it back to its original form?

• Restricted matching, e.g., certain pattern variables might be allowed to match only numeric constants.

• Evaluation of expressions in the output pattern, e.g. to fold constants.

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