code: private void checkVariableRange(int ch, String rule, int start) {
          if (ch >= curData.variablesBase && ch < variableLimit) {
              syntaxError("Variable range character in rule", rule, start);
          }
      }
docs: Assert that the given character is NOT within the variable range.
  If it is, signal an error.  This is neccesary to ensure that the
  variable range does not overlap characters used in a rule.