Excerpt from A High Level Real-Time Programming Language
User-defined functions Whldl are invoked on the right side of a bind or follow statement must be such that it makes sense to think of evaluating them instantaneously and continously. For example, if the function F had a side-effect of incrementing a global variable X by 1, then the statement would have to increment X infinitely often, Whld'l is not ameptable. We therefore define an instantaneous function as one where the only variables accessed are the argu ments (passed by value) and loml variables; and we require that only instantaneous functions be used on the right-hand side of a bind or follow statement.