code: public String createNewVariableOnSolver(final String prefix) {
          final int index = this.name2idx.size() + 1;
          final String varName = prefix + "_" + index;
          this.name2idx.put(varName, index);
          this.idx2name.put(index, varName);
          return varName;
      }
docs: Creates a new variable on the solver and returns its name.
  @param prefix the prefix of the variable name
  @return the name of the new variable