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.
See section 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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