code: public final boolean addConstraintNoPropagation(Constraint c) {
  		this.skipPropagation = true;
  		this.addConstraints(c);
  		this.skipPropagation = false;
  		return true;
  	}
docs: Add a constraint between {@link Variable}s, without propagation.
  @param c The constraint to add.
  @return <code>true</code>.