code: private boolean eachVariableOccursOnlyOnce() {
          int lit;
          for (int i = 0; i < this.addedlits.size(); i++) {
              lit = this.addedlits.get(i);
              assert marked(lit) == 0;
              assert marked(-lit) == 0;
          }
          boolean res = true;
          for (int i = 0; i < this.addedlits.size(); i++) {
              lit = this.addedlits.get(i);
              if (marked(lit) != 0 || marked(-lit) != 0) { res = false; } else { mark(lit); }
          }
          unmark();
          return res;
      }
docs: Returns {@code true} if each variable occurs only once in the current clause, {@code false} otherwise.
  @return {@code true} if each variable occurs only once in the current clause