database( {
	mother( Child:string, Mother:string ),
      	father( Child:string, Father:string ),
	orphan( Person:string ),
	french( Person:string )
} ).


%
% meta_pred_list( $ConstraintList, $VariableList, BindingList )
%
% ConstraintList = a list of literals in the form of functors or
%		   a list of lists of literals
% VariableList   = a list of variables to be returned
% BindingList    = a list of binding corresponding to each variable
%                  in VariableList
%
% The functor ldl_var(Term) used in ConstraintList and VariableList
% of the meta_pred_list/3 is interpreted as a LDL variable.
%

/*
 Run the following queries

query call( parent(ldl_var(1), ldl_var(2)), [ ldl_var(1), ldl_var(2) ], Binding )
query call( parent(liz, ldl_var(1)), [ ldl_var(1) ], Binding )
query meta_parent(X, Y)
query meta_parent(liz, X)

*/

export call($Pred, $VarList, Binding).
export meta_parent(X, Y).
export meta_parent($X, Y).
export meta_parent(X, $Y).
export meta_parent($X, $Y).

%
% call($Pred, $VarList, Binding)
%

call(Pred, VarList, Binding) <-
	meta_pred_list( [Pred], VarList, Binding ). 

%
% meta_parent(X, Y)
%

meta_parent(X, Y) <-
	meta_pred_list( [ [father(ldl_var(1), ldl_var(2))],
			  [mother(ldl_var(1), ldl_var(2))] ],
		        [ ldl_var(1), ldl_var(2) ], [ X, Y ] ).

%
% parent(Child, Parent)
%       Parent is either the mother or the father of Child.
%
parent(Child, Father) <-
	father(Child, Father).
parent(Child, Mother) <-
	mother(Child, Mother).