How does the '|' character in prolog append rather than split a list?

My example is using the append functor:

append([],L,L). 
append([H|T],L2,[H|L3])  :-  append(T,L2,L3).

//append([123],[abc],L3). this is the query

What really confuses me is the [H|L3] list. Form what I read, that takes off the head, so how is that appending it to the list. When it recurses through the list, and starts coming back, how is that appended? Thanks in advance.

Solution

You can use trace/0 in those cases when you wonder what the execution looks like.

Here is an overview of it.

You call append([1, 2, 3], [a, b, c], L3).
[1, 2, 3] is not empty, the second clause is applied
It calls append([2, 3], [a, b, c], L4). and remembers that the head of L3 is like the head of [1, 2, 3] and that the tail of L3 is L4
[2, 3] is not empty, the second clause is applied
It calls append([3], [a, b, c], L5). and remembers that the head of L4 is like the head of [2, 3] and that the tail of L4 is L5
[3] is not empty, the second clause is applied
It calls append([], [a, b, c], L6). and remembers that the head of L5 is like the head of [3] and that the tail of L5 is L6
[] unifies with [], the first clause is applied
It says that L6 unifies with [a, b, c]

Now Prolog has all the informations to answer you:

L6 = [a, b, c],
L5 = [3|L6],
L4 = [2|L5],
L3 = [1|L4].

will allow Prolog to conclude L3 = [1, 2, 3, a, b, c].

Hope it helped.