_if		_if boolean-expression cmd ["else" cmd].  This command is used
 		to perform a macro based on the evaluation of a boolean
 		expression. If the boolean expression is true, the first
 		command is executed and if it is not, and there is an "else"
 		clause, the second is executed. For example:
                 _if '! me & you' '@do something' else '@do nothing'
                 When using _if, you must be careful of quoting.