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

• A rule is a leaf or a compound term.
• A leaf term (op) is a word.
• A compound term is a word followed by an argument list.
• A word has one letter or more.
• One-letter words are variables.
• Whitespace is ignored.
• Case-sensitive (a and A are different).

Grammar

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