A selection directive selects source text to compile.
Syntax
$IF boolean_static_expression $THEN text [ $ELSIF boolean_static_expression $THEN text ]... [ $ELSE text $END ]
The selection directive evaluates the BOOLEAN static expressions.
If one expression has the value TRUE, its text is compiled, the remaining expressions are not evaluated.
If no expression has the value TRUE, then if $ELSE is present, its text is compiled; otherwise, no text is compiled.