update doc

    not modify the number.

* later                                                               :entry:

** 2017-01-27 the whole FSM can be made persistent across sessions.

not only trail.

- load FSM data on page init.

- save FSM data on each call to FSM functions.

The usefulness is doubtful. I will leave it.

** 2017-01-26 make the whole thing a polymer web component

<rpn-calculator trail="true"></rpn-calculator>

* current                                                             :entry:

** 

** 2017-01-27 feature: long press backspace just delete the current number.

** 2017-01-27 add some visual feedback when clicking a button. like in material design.

** 2017-01-27 UI problem: when input a very large number, #number-display and keyboard will grow.

should set a max-width on #number-display.

overflow: auto

doesn't work with <td> in firefox.

** 2017-01-26 make it work offline, add sw.js

** 2017-01-26 make undo work

* done                                                                :entry:

** 2017-01-26 make basic things work.

- DONE draw the keyboard using HTML and CSS

- DONE make basic number input and arithmetic work

- DONE make DUP and SWP work

- DONE make trail work

** 2017-01-26 make trail persistent

trail is a bounded fifo queue, it just store recent 1k entries.

when multiple instances of calculator is working. they all write to a single

trail cache. it's not a problem.

** 2017-01-27 make trail work

- when is trail being updated?

  - when a number is committed to number stack. return.

    it's because shortKeyNames key should be key name string literal, not the

    js constant identifier.

** 2017-01-26 make trail persistent

trail is a bounded fifo queue, it just store recent 1k entries.

when multiple instances of calculator is working. they all write to a single

trail cache. it's not a problem.

** 2017-01-26 make it work offline, add sw.js

** 2017-01-26 make undo work

** 2017-01-26 make basic things work.

- DONE draw the keyboard using HTML and CSS

- DONE make basic number input and arithmetic work

- DONE make DUP and SWP work

- make trail work

* done                                                                :entry:

** 2017-01-27 design a layout for mobile.

- layout on mobile:

  stack

** 2017-01-26 make basic number input and arithmetic work

- DONE make event work.

  - TODO add some visual feedback when clicking a button. like in material design.

  - MOVED add some visual feedback when clicking a button. like in material design.

- DONE design data structure for the calculator.

- DONE add unit test for the data structure and functions.

- DONE make number input and simple calculation work.

** 2017-01-26 draw the keyboard using HTML and CSS

* wontfix                                                             :entry:

** 2017-01-27 feature: long press backspace just delete the current number.

there is no long press mouse event.

won't add this feature.