User Defined Functions are the same as any other rule, except the user defines parameters with a special notation. The system will not search for a variable beginning with “_” (underscore). Instead, the user must pass the value to use as a parameter.
Definition:
product(_x,_y) := _x * _y
The user may call product anytime the rule is available:
product(4,5) will return 20;
product(3,2) will return 6.