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nonground(+N, ?Term, -VarList)

Succeeds if Term contains at least N different variables, and returns N of them in the list VarList.
N
Integer.
Term
Prolog term.
VarList
List or variable.

Description

Used to test whether Term contains at least N different variables. The argument VarList is unified with a list of exactly N of those variables. If Term contains more than N variables, it is not further specified which ones will be in the list and in which order. As usual, attributed variables are also considered variables.

Note that this predicate is a generalisation of nonground/1 and nonground/2 which could be written as: 

    nonground(Term) :- nonground(1, Term, _).
    nonground(Term, Var) :- nonground(1, Term, [Var]).

This predicate can handle cyclic terms.

Modes and Determinism

Fail Conditions

Fails if Term contains less than N variables

Exceptions

(4) instantiation fault
N is not instantiated.
(5) type error
VarList instantiated but not to a list.
(6) out of range
N is not positive.

Examples

Success:
    nonground(1, Term, L).       % gives L = [Term]
    nonground(1, f(a,B,c), L).   % gives L = [B]
    nonground(2, [X,Y,Z], L).    % gives L = [Y,X]
    nonground(2, [X,Y,Z], L).    % gives L = [Y,X]
    nonground(1, s(X{a}), L).    % gives L = [X{a}]

Fail:
    nonground(1, atom, L).
    nonground(2, f(a,B,c), L).
    nonground(2, [X,X,X], L).

See Also

nonground / 1, nonground / 2, nonvar / 1, var / 1