plus: function() {
    this.addPart_();
    this.updateMinus_();
  },
  minus: function() {
    if (this.elseIfCount_ == 0) {  // Handles programmatic calling (just in case).
      return;
    }
    this.removePart_();
    this.updateMinus_();
  },
  // To properly keep track of indices we have to increment before/after adding
  // the inputs, and decrement the opposite.
  // Because we want our first elseif to be IF1 (not IF0) we increment first.
  addPart_: function() {
    this.elseIfCount_++;
    this.appendValueInput('IF' + this.elseIfCount_)
        .setCheck('Boolean')
        .appendField(Blockly.Msg['CONTROLS_IF_MSG_ELSEIF']);
    this.appendStatementInput('DO' + this.elseIfCount_)
        .appendField(Blockly.Msg['CONTROLS_IF_MSG_THEN']);

    // Handle if-elseif-else block.
    if (this.getInput('ELSE')) {
      this.moveInputBefore('ELSE', /* put at end */ null);
    }
  },
  removePart_: function() {
    this.removeInput('IF' + this.elseIfCount_);
    this.removeInput('DO' + this.elseIfCount_);
    this.elseIfCount_--;
  },
  updateMinus_: function() {
    var minusField = this.getField('MINUS');
    if (!minusField) {  // Assume we want to add one.
      this.topInput_.insertFieldAt(1, new FieldMinus(), 'MINUS');
    } else if (!this.elseIfCount_) {
      this.topInput_.removeField('MINUS');
    }
  }


// Mutator helper function to add the plus.
const controlsIfHelper = function() {
  this.topInput_ = this.getInput('IF0');
  this.topInput_.insertFieldAt(0, new FieldPlus(), 'PLUS');
};