Syntax of Sentences as Terms

     sentence          --> module : sentence
                        |  list
                              { where list is a list of sentence }
                        |  clause
                        |  directive
                        |  query
                        |  grammar-rule
     
     clause            --> rule | unit-clause
     
     rule              --> head :- body
     
     unit-clause       --> head
                              { where head is not otherwise a sentence }
     
     directive         --> :- body
     
     query             --> ?- body
     
     head              --> module : head
                        |  goal
                              { where goal is not a variable }
     
     body             --> module : body
                        |  body -> body ; body
                        |  body -> body
                        |  \+ body
                        |  body ; body
                        |  body , body
                        |  goal
     
     goal              --> term
                              { where term is not otherwise a body }
     
     grammar-rule      --> gr-head --> gr-body
     
     gr-head           --> module : gr-head
                        |  gr-head , terminals
                        |  non-terminal
                              { where non-terminal is not a variable }
     
     gr-body           --> module : gr-body
                        |  gr-body -> gr-body ; gr-body
                        |  gr-body -> gr-body
                        |  \+ gr-body
                        |  gr-body ; gr-body
                        |  gr-body , gr-body
                        |  non-terminal
                        |  terminals
                        |  gr-condition
     
     non-terminal      --> term
                              { where term is not otherwise a gr-body }
     
     terminals         --> list | string
     
     gr-condition      --> ! | { body }
     
     module            --> atom