code: protected boolean locked(final MSClause c) {
      return value(c.get(0)) == Tristate.TRUE && v(c.get(0)).reason() != null && v(c.get(0)).reason() == c;
    }
docs: Returns {@code true} if the given clause is locked and therefore cannot be removed, {@code false} otherwise.
  @param c the clause
  @return {@code true} if the given clause is locked