back

back undoes the last line that changed the world. Up to 64 lines can be undone.

Lines that changed nothing, such as words, help or a failed line, aren't recorded, so back skips them.

Several at once

back back back

rewinds three lines. Everything else on a line containing back is discarded, so [ 1 ] :x back doesn't define x.

Scope

back restores the stack and the tab's definitions, the same state a transaction protects. Root edits, printed text, other tabs and anything written to memory, a port or the screen are not affected, so back can't undo a poke.

On an installed disk

A rewind is saved like any other change, but the history isn't, so after a reboot back has nothing to return to (saving).

The Up arrow at the prompt recalls the text of a line, not its effect.