7.21 Even and Odd Operators

An operator can be declared to be even or odd in its first argument by the declarations even and odd respectively. Expressions involving an operator declared in this manner are transformed if the first argument contains a minus sign. Any other arguments are not affected. In addition, if say f is declared odd, then f(0) is replaced by zero unless f is also declared non zero by the declaration nonzero. For example, the declarations

        even f1; odd f2;

mean that

        f1(-a)    ->    f1(a)
        f2(-a)    ->   -f2(a)
        f1(-a,-b) ->    f1(a,-b)
        f2(0)     ->    0.

To inhibit the last transformation, say nonzero f2;.

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