private void onEquals() {
   String text = mFormulaEditText.getCleanText();
   if (mCurrentState == CalculatorState.INPUT) {
     switch (mEqualButton.getState()) {
       case EQUALS:
         setState(CalculatorState.EVALUATE);
         mEvaluator.evaluate(text, this);
         break;
       case NEXT:
         mFormulaEditText.next();
         break;
     }
   } else if (mCurrentState == CalculatorState.GRAPHING) {
     setState(CalculatorState.EVALUATE);
     onEvaluate(text, "", INVALID_RES_ID);
   }
 }