Last X:

The Last X register is an invisible one that will store the last value that was used from the X register in a mathematical operation. The value is recalled by pressing “l” (lowercase ell) or clicking the LastX button.

Example:

3 Enter 4 +

This will yield the answer of 7 in the X register, and the Last X register will contain the last value from the X register, namely, 4.

This can be used as a primitive undo function, provided you know which operation you did last, and that there is an inverse operation. For example, immediately following the example above, you can enter:

l - l (ell, minus, ell)

This will restore the stack to the state it had just prior to the addition. (For a complete undo, use Alt-Backspace or Ctrl-Z).