Restrictions on the Use of References to Functions If a reference to a function appears as a symbol in a ground expression, the function must be capable of taking as input any ground expression and is allowed to return any ground expression. restrictions on the use of references to functions

Namely, if a construct &Fname, which is a reference to the function Fname, appears in a pattern expression or in a result expression, the function Fname must be declared as either $func Fname e = e or $func? Fname e = e.