!IF construct may be used inside a macro body to allow for
conditional expansion. It takes the following forms:
!IF (expression) !THEN true-expansion !IFEND !IF (expression) !THEN true-expansion !ELSE false-expansion !IFEND
When expression evaluates to true, the macro processor expands true-expansion; otherwise, it expands false-expansion, if it is present. The macro processor considers quoted or unquoted ‘0’ to be false, and anything else to be true.