code: func (a *Assertions) Len(object interface{}, length int, msgAndArgs ...interface{}) bool {
  	if h, ok := a.t.(tHelper); ok {
  		h.Helper()
  	}
  	return Len(a.t, object, length, msgAndArgs...)
  }
docs: // Len asserts that the specified object has specific length.
  // Len also fails if the object has a type that len() not accept.
  //
  //    a.Len(mySlice, 3)