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The substitute command is a generic interface to the rewrite engine in Pynac/GiNaC. You can replace an arbitrary subexpression, also containing wildcards (see the documentation of the match() method of symbolic expressions), with another expression.

Consider the case where a subexpression occurs in several places and you have a better formula or value for it derived from other sources. You can just replace the existing subexpression with the new formula using the substitute() function.