A Prolog Subset.

A grammar for a small Prolog language is given below; it follows the grammar for Horn clause form closely. Variable names begin with a capital letter. It simplifies matters to make a program consist of zero or more rules followed by a single query; in other systems queries can be interspersed with the rules. Note also that most Prologs use `:-' instead of `<=' and use `,' instead of `and' to separate atoms.

<Program>    ::= [ <rules> ] <query>

<rule>       ::= <atom> . | <atom> <= <literals> .
<rules>      ::= <rule> [ <rules> ]
<query>      ::= ? <literals> .
<literal>    ::= <atom> | not <atom>
<literals>   ::= <literal> [ and <literals> ]
<atom>       ::= <ident> | <ident> ( <terms> )
<term>       ::= <ident> | <numeral> |             -- constants
                 <ident> ( <terms> ) |             -- functions
                 <IDENT>                           -- variables
<terms>      ::= <term> [, <terms> ]

 --- Prolog-S Syntax. ---