plus: function() {
    this.addArg_();
    Blockly.Procedures.mutateCallers(this);
  },
  minus: function(argId) {
    if (!this.argIds_.length) {
      return;
    }
    this.removeArg_(argId);
    Blockly.Procedures.mutateCallers(this);
  },
  addArg_: function(opt_name, opt_varId) {
    if (!this.arguments_.length) {
      var withField = new Blockly.FieldLabel(
          Blockly.Msg['PROCEDURES_BEFORE_PARAMS']);
      this.getInput('TOP')
          .appendField(withField, 'WITH');
    }

    var name = opt_name || Blockly.Variables.generateUniqueNameFromOptions(
        Blockly.Procedures.DEFAULT_ARG, this.arguments_);
    var variable = Blockly.Variables.getOrCreateVariablePackage(
        this.workspace, opt_varId, name, '');
    var argId = Blockly.utils.genUid();

    this.addVarInput_(name, argId);
    this.moveInputBefore(argId, 'STACK');

    this.arguments_.push(name);
    this.varIds_.push(variable.getId());
    this.argumentVarModels_.push(variable);
    this.argIds_.push(argId);
  },
  removeArg_: function(argId) {
    if (!this.getInput(argId)) {
      return;
    }
    // TODO: My life would be so much happier if this returned a boolean instead
    //  of throwing an error. I'm saying at least 15% more joy.
    this.removeInput(argId);
    if (this.arguments_.length == 1) {  // Becoming argumentless.
      this.getInput('TOP').removeField('WITH');
    }

    var index = this.argIds_.indexOf(argId);
    this.arguments_.splice(index, 1);
    this.varIds_.splice(index, 1);
    this.argumentVarModels_.splice(index, 1);
    this.argIds_.splice(index, 1);
  },