Db        . Database ::= [Clause] ;

Fact      . Clause ::= Predicate ;
Rule      . Clause ::= Predicate ":-" [Predicate] ;
Directive . Clause ::= ":-" [Predicate] ;

APred    . Predicate ::= Atom ;
CPred    . Predicate ::= Atom "(" [Term] ")" ;

TAtom    . Term ::= Atom ;
VarT     . Term ::= Var ;
Complex  . Term ::= Atom "(" [Term] ")" ;
TList    . Term ::= List ;

Atm      . Atom ::=  LIdent ;
EAtm     . Atom ::=  "\'" Ident "\'";

V        . Var ::= UIdent ;
A        . Var ::= Wild ;

Empty . List ::= "[" "]";
Enum  . List ::= "[" [Term] "]" ;
Cons  . List ::= "[" [Term] "|" List "]" ;
ConsV . List ::= "[" [Term] "|" Var "]" ;

-- List definitions

terminator Clause "." ;
separator nonempty Predicate "," ;
separator nonempty Term "," ;

-- Token definitions

token UIdent (upper (letter | digit | '_')*) ;
token LIdent (lower (letter | digit | '_')*) ;
token Wild   '_' char* ;