Undoing Mistakes
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The shift-`U' key (`calc-undo') undoes the most recent operation.  If
that operation added or dropped objects from the stack, those objects
are removed or restored.  If it was a "store" operation, you are
queried whether or not to restore the variable to its original value.
The `U' key may be pressed any number of times to undo successively
farther back in time; with a numeric prefix argument it undoes a
specified number of operations.  The undo history is cleared only by
the `q' (`calc-quit') command.  (Recall that `M-# c' is synonymous
with `calc-quit' while inside the Calculator; this also clears the
undo history.)

Currently the mode-setting commands (like `calc-precision') are not
undoable.  You can undo past a point where you changed a mode, but you
will need to reset the mode yourself.

The shift-`D' key (`calc-redo') redoes an operation that was
mistakenly undone.  Pressing `U' with a negative prefix argument is
equivalent to executing `calc-redo'.  You can redo any number of
times, up to the number of recent consecutive undo commands.  Redo
information is cleared whenever you give any command that adds new
undo information, i.e., if you undo, then enter a number on the stack
or make any other change, then it will be too late to redo.

The `M-RET' key (`calc-last-args') is like undo in that it restores
the arguments of the most recent command onto the stack; however, it
does not remove the result of that command.  Given a numeric prefix
argument, this command applies to the `n'th most recent command which
removed items from the stack; it pushes those items back onto the
stack.

The `K' (`calc-keep-args') command provides a related function to
`M-RET'.  See Stack and Trail.

It is also possible to recall previous results or inputs using the trail.
See Trail Commands.

The standard Emacs `C-_' undo key is recognized as a synonym for `U'.