Protected error while using PrependTo at Flatten?

Questions
1

Hello,
I run these codes and see an error:

a = PrependTo[Flatten[{{1, 2}}], {3}]

The error is:

Tag Flatten in Flatten[{{1,2}}] is Protected.

It is weird.

2

Try Prepend instead of PrependTo

a = Prepend[Flatten[{{1, 2}}], {3}]
3

Okay. Thanks a lot.

4

If you are wondering why there is a difference, compare

Attributes@PrependTo
(* {HoldFirst, Protected} *)

Attributes@Prepend
(* {Protected} *)
5

How to understand HoldFirst and Protected? I see them in documentation, but they are too abstract.

6

(1) This forum is not intended to be a replacement for reading relevant documentation. It seems quite unlikely that you checked Help > Wolfram Documentation > PrependTo. And that is a prerequisite for posting a question like this.

(2) The documentation for PrependTo is quite clear on why the first argument needs to be something that can take an assignment. You cannot assign a value to Flatten[{{1, 2}}].

(3) It might be easier to understand this related example.

In[3]:= PrependTo[a, 3]

During evaluation of In[3]:= PrependTo::rvalue: a is not a variable with a value, so its value cannot be changed.

Out[3]= PrependTo[a, 3]
7

Thanks for your reminder. It’s my fault. I will check that.