code: public final Variable getVariable(int variableNum) {
      int index = getVariableIndex(variableNum);
      if (index >= 0) {
        return vars[index];
      } else {
        return null;
      }
    }
docs: Get the variable referenced by a particular variable number.
  Returns {@code null} if no variable exists with that number.
  
  @param variableNum
  @return