* COMMENT -*- mode: org -*-
#+Date: 2017-01-26
Time-stamp: <2017-01-28>

* an RPN calculator implemented in HTML and client side javascript
It can be used offline on platforms that support service worker.

It use a compute and UI model similar to emacs calc-mode.

* UI
--- stack ---    Keyboard            |   --- trail ---
                                     |
                 [            ]      |
                                     |
                 7   8   9    <      |
                                     |
                 4   5   6    *      |
                                     |
                 1   2   3    -      |
                                     |
                 0   /   .    +      |
                                     |
                SWP UNDO     RET     |

* features
- totally event based
- virtual keyboard
- everything on the top of stack adds a record in trail window
- show error when an event is invalid. not enough element, division by zero etc.
- partial number input is in virtual display, press RET to send number to stack.
- you can duplicate top value on stack using RET
- you can swap top 2 values on stack using SWP
- you can delete top value on stack using ←
- you can undo using UNDO