Start with a clean sheet of paper.
The display shows the
stack, which is your work area. Currently the
stack is empty.
to write 15 in the
Note that the stack moves up to make room for the command line, so
only three stack levels are displayed.
Put 15 on the stack.
The number goes in
as indicated by 1 : at the left. Note
that the command line disappears, so four stack levels are displayed
to write 23 in the command line.
Put 23 in level 1.
The number 15, which was in level 1, is lifted to level 2.
1: Getting Started