Returns the name of a called function. The result is a representation of the name of the applied function F/A, if Node represents a function application "F(X_1, ..., X_A)". If the function is not explicitly named (i.e., F is given by some expression), only the arity A is returned.
The evaluation throws syntax_error if Node does not represent a well-formed application expression.
See also: analyze_function_name/1.