code: func (a *Assertions) Lenf(object interface{}, length int, msg string, args ...interface{}) {
  	if h, ok := a.t.(tHelper); ok {
  		h.Helper()
  	}
  	Lenf(a.t, object, length, 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.
  //
  //    a.Lenf(mySlice, 3, "error message %s", "formatted")