code: func Lenf(t TestingT, object interface{}, length int, msg string, args ...interface{}) bool {
  	if h, ok := t.(tHelper); ok {
  		h.Helper()
  	}
  	return Len(t, object, length, append([]interface{}{msg}, args...)...)
  }
docs: // Lenf asserts that the specified object has specific length.
  // Lenf also fails if the object has a type that len() not accept.
  //
  //    assert.Lenf(t, mySlice, 3, "error message %s", "formatted")