code: public BDD exists(final Collection<Variable> variables) {
      final BDD resBDD = this.factory.build(this.factory.getF().and(variables));
      return new BDD(this.factory.underlyingKernel().exists(index(), resBDD.index()), this.factory);
    }
docs: Existential quantifier elimination for a given set of variables.
  @param variables the variables to eliminate
  @return the BDD with the eliminated variables