3.5.4 Exchange

This transformation performs the trivial but otherwise tedious task of exchanging the expressions on either side of an equation. The transformation can also be applied to relational expressions.

For commutative relational operators, simply swapping the operands is sufficient. But for non-commutative operators, the operator must be changed as well. So for a=b, Exchange produces b=a but for a>b, Exchange produces b≤a.