Id              ::= [a-z] [A-Za-z0-9_]*
Variable        ::= [A-Z] [A-Za-z0-9_]*
StringConstant  ::= \"([^\"] | \\\")\"
SymbolicConstant    ::= [a-z] [A-Za-z0-9_]*
AnonymousVariable   ::= "_"
Number          ::= 0 | [1-9][0-9]*
VariableTerm    ::= Variable | AnonymousVariable
GroundTerm      ::= SymbolicConstant | StringConstant | "-"? Number
BasicTerm       ::= GroundTerm | VariableTerm
BasicTerms      ::= BasicTerm ("," BasicTerm)*
ArithOp         ::= "+" | "-" | "*" | "/"

Term            ::= Id ("(" Terms? ")")?
                    | Number  
                    | StringConstant
                    | Variable
                    | AnonymousVariable
                    | "(" Term ")"
                    | "-" Term
                    | Term ArithOp Term
Terms           ::= Term ("," Term)*

BinOp           ::= "<" | "<=" | "=" | "!=" | ">" | ">="

BuiltinAtom     ::= Term BinOp Term
ClassicalLiteral    ::= "-"? SymbolicConstant ("(" Terms? ")")? 

NafLiteral      ::= "not"? ClassicalLiteral | BuiltinAtom
NafLiterals     ::= NafLiteral ("," NafLiteral)*

WeightAtLevel   ::= Term ("@" Term)? ("," Terms)?

AggregateFunction   ::= "#count" | "#sum" | "#max" | "#min"
AggregateElement    ::= BasicTerms? (":" NafLiterals?)?
AggregateElements   ::= AggregateElement (";" AggregateElement)*
Aggregate       ::= (Term BinOp)? AggregateFunction "{" AggregateElements? "}" (BinOp Term)?    

ChoiceElement   ::= ClassicalLiteral (":" NafLiterals?)?
ChoiceElements  ::= ChoiceElement (";" ChoiceElement)*
Choice          ::= (Term BinOp)? "{" ChoiceElements? "}" (BinOp Term)?

Disjunction     ::= ClassicalLiteral ("|" ClassicalLiteral)* 

Body            ::= (Body ",")? (NafLiteral | "not"? Aggregate)
Head            ::= Disjunction | Choice

Statement       ::= ":-" Body? "."
                    | Head (":-" Body?)? "."
                    | ":~" Body? "." "[" WeightAtLevel "]"

Query           ::= ClassicalAtom "?"
Statements      ::= Statement+

Program         ::= Statements? Query?