$$ Subrule-Invocation ::= Rule-Name "(" Expression {"," Expression} ")" . $$ Rule-Name ::= Identifier .
A subrule is invoked when an expression ``subrule (value1,
value2, )'' is evaluated. The expression yields the value that is
returned by the return statement in the subrule. The number of parameters
in a subrule invokation must match the number of parameters in the subrule
definition.
There is a number of default subrules which are predefined. They are called functions and they all take one parameter only.