<expression1> onlyif <expression2>
Boolean true or false, depending on the evaluation of <expression2>.
onlyif is designed as a short circuit within more complex expressions, particularly function definitions. Since Polish notation is evaluated from right to left, an onlyif expression will be evaluated first if it is at the end of a line. If <expression2> returns false, <expression1> is not evaluated, and the result of the overall expression is false.