calc: Rewrite Rules
11.11 Rewrite Rules
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The ‘a r’ (‘calc-rewrite’) [‘rewrite’] command makes substitutions in a
formula according to a specified pattern or patterns known as “rewrite
rules”. Whereas ‘a b’ (‘calc-substitute’) matches literally, so that
substituting ‘sin(x)’ with ‘cos(x)’ matches only the ‘sin’ function
applied to the variable ‘x’, rewrite rules match general kinds of
formulas; rewriting using the rule ‘sin(x) := cos(x)’ matches ‘sin’ of
any argument and replaces it with ‘cos’ of that same argument. The only
significance of the name ‘x’ is that the same name is used on both sides
of the rule.
Rewrite rules rearrange formulas already in Calc’s memory. 
Syntax Tables, to read about “syntax rules”, which are similar to
algebraic rewrite rules but operate when new algebraic entries are being
parsed, converting strings of characters into Calc formulas.
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