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. ---