code: @Override
      protected String pbLhs(final Literal[] operands, final int[] coefficients) {
          assert operands.length == coefficients.length;
          final List<Literal> sortedOperands = new ArrayList<>();
          final List<Integer> sortedCoefficients = new ArrayList<>();
          final List<Literal> givenOperands = Arrays.asList(operands);
          for (final Variable v : this.varOrder) {
              final int index = givenOperands.indexOf(v);
              if (index != -1) {
                  sortedOperands.add(v);
                  sortedCoefficients.add(coefficients[index]);
              }
          }
          for (final Literal givenOperand : givenOperands) {
              if (!sortedOperands.contains(givenOperand)) {
                  sortedOperands.add(givenOperand);
                  sortedCoefficients.add(coefficients[givenOperands.indexOf(givenOperand)]);
              }
          }
          final StringBuilder sb = new StringBuilder();
          final String mul = pbMul();
          final String add = pbAdd();
          for (int i = 0; i < sortedOperands.size() - 1; i++) {
              if (sortedCoefficients.get(i) != 1) {
                  sb.append(sortedCoefficients.get(i)).append(mul).append(sortedOperands.get(i)).append(add);
              } else {
                  sb.append(sortedOperands.get(i)).append(add);
              }
          }
          if (sortedOperands.size() > 0) {
              if (sortedCoefficients.get(sortedOperands.size() - 1) != 1) {
                  sb.append(sortedCoefficients.get(sortedOperands.size() - 1)).append(mul).append(sortedOperands.get(sortedOperands.size() - 1));
              } else {
                  sb.append(sortedOperands.get(sortedOperands.size() - 1));
              }
          }
          return sb.toString();
      }
docs: Returns the sorted string representation of the left-hand side of a pseudo-Boolean constraint.
  
  @param operands     the literals of the constraint
  @param coefficients the coefficients of the constraint
  @return the sorted string representation