todo.html

<a href="help.html">Done</a>
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* More constants, see: https://en.wikipedia.org/wiki/List_of_physical_constants
* More formulas, see: https://www.madison-lake.k12.oh.us/userfiles/680/Classes/16192/IED-Review%20Engineering%20Formula%20Sheet.pdf
* program button for units/constants, show on list, press 'program', list item and finally key to replace
* method of adding new units, constants or formulas by user,
-   integrity checks (no duplicates, bad shortUnits) for units and constants
* formula filter button on list screen, reset shows all, allows filtering formulas shown
-   shows number of matching formulas under each group
-   hide button on each group, sticky
* Add 'Backspace' to keyboard and calculator keyboard
* Fix sizes of values and units, excesive padding and centering on mobile

App like features:
* 'legend' group at start of list for units and constants for keys in keyboard
* animate hiding keyboard when lists or units is shown, slide new keys in or flip animation
* typescript formulas: https://jkorpela.fi/math/ or http://docs.mathjax.org/en/latest/ (partially doing so now with unicode)
* progressive web app: https://medium.com/@firt/progressive-web-apps-on-ios-are-here-d00430dee3a7

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