DEFINE FUNCTION

        DEFINE FUNCTION NAME(X,Y,Z) Expression(X,Y,Z)

    defines an arithmetic user function of several  variables.  The  maximum
    number  of  variables is 4. The function definition can reference any of
    the  known  mathematic  operators  and  intrinsic  or  program   defined
    functions, but not previously defined user functions (i.e. user function
    definition is not recursive).