Two small fixes.

• DependencyHelper.why(): Use LinkedHashSet instead of TreeSet. -- The C type is not constrained as comparable. Alternative change would be to constrain it with C extends Comparable<C>.
• AbstractPBClauseCardConstrDataStructure.constraintFactory(): I believe the conditional should compare with null and return Constr.TAUTOLOGY. At least that makes some sense because v cannot ever be equal to Constr.TAUTOLOGY.