code: function addTypeFunction (typeName, funcName, func) {
    if (!stringValue(typeName)) {
      throw new Error('Invalid "typeName" argument, must be String')
    } else if (!_.isFunction(func) && !stringValue(func)) {
      throw new Error('Invalid func argument, must be function')
    }
    _.set(this.backing, [ typeName, `_${funcName}` ], func)
    return this
  }
docs: Adds a type function to the backing
  @param typeName
  @param funcName
  @param func
  @returns {addTypeFunction}