Several issues remain to be addressed:

• Efficiency: If there are many patterns, we want to organize them 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., one might wish to restrict certain pattern variables to match only numeric constants.

• Evaluation of expressions in the output pattern: if a transformation produces (+ 2 3), it could be evaluated to 5.

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