Some examples

Son(John,Bill)
  Is John Bill's son?
Son(X,Bill)   
  List all … sons of Bill
Son(John,X)   
… persons of which John is a son
Son(X,Y)      
… son relationships

Rule Syntax

Grammar

Rule     ::= Op | Compound
Op       ::= Letter+
Compound ::= Op "(" Rule ( "," Rule )* ")"
Letter   ::= [A-Za-z]