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This is a useful heuristic, quite effective, but not widely
applicable.
The idea is best illustrated with an example.
Suppose that our expression to match is a rational polynomial
in *x*.
Suppose that the pattern has terms in *x* and in *e*^{x}.
Since we know that no polynomial in *x* is equal to any
polynomial in *e*^{x}, then then only possible matching is
one which eliminates all appearances of *e*^{x} in the pattern.

*Gaston Gonnet*

*1999-07-04*