PLEASE PLEASE an Undo

[eried]

Undo ONLY needs to work in the input text boxes. I don't think anyone is dreaming a whole complete constant snapshot based-UNDO.

Up to N steps, just save the input differential after 'some actions'.

Example: after an operand,

Code:

2x+
*UNDO*
2x
*UNDO*

*REDO*
2x

Example: after a newline/Del/Clear,

Code:

2x+
*DEL*

*UNDO*
2x+

Menu key suits perfectly for Redo/Undo commands, and ON+Z, ON+Y as the key combinations