Any function argument can be protected from evaluation by an exclamation mark (!) before the argument name in the function's argument list. For example,

function f (x, !y)

creates a function with two mandatory arguments , the second of which will not be evaluated when it is called. If this function were called as f (2+2, 3+3) then x would have the value of 4, and y would have as its value the expression 3+3. y could be evaluated using eval(y) to produce the value 6.