applies a rule or list of rules in an attempt to transform each subpart of an expression expr.
- ReplaceAll looks at each part of expr, tries all the rules on it, and then goes on to the next part of expr. The first rule that applies to a particular part is used; no further rules are tried on that part, or on any of its subparts.
- ReplaceAll applies a particular rule only once to an expression.
- returns expr if none of the rules apply.
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