don't lose the user's work.
two ways that can happen:
 - they fail to write before quitting
   - so write a '.unsaved' file on exit
 - they write when they shouldn't
   - so keep at least one older version around
also, give more feedback -- messages -- on writing
 
arrow keys
 
when editing a cell: left and right arrow, C-a, C-e, clear
make = a keystroke command
 
grid bigger than the screen
 
aggregates over cell ranges
 
detect screen dimensions
 
let overwide cell display overflow into any empty cells adjacent to
the right
 
color-code formulas/values/labels/plaints
 
highlight and navigate dependents/dependencies of the current cell
 
make global settings from a spreadsheet: search for a setting-name and
pick the value out of the cell next to it
 
boring practicality: 
  use decimal arithmetic
  numeric decimal formatting widths
 
table lookup (for tax tables)
conditional expressions
 
"Minimal-keystroke formula entry: type "1.1*" then move the cursor
then type "-1" to enter a formula. The goal here was to make it worth
using the first time you needed an answer (instead of a calculator and
paper) in a way that would let you benefit the next time by just
changing a few values and recalculating. If the input style did not
let you "teach" the computer by doing the calculation, people may not
have used it."
http://j2k.naver.com/j2k.php/heightK/www.bricklin.com/firstspreadsheetquestion.htm
 
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a pivoting spreadsheet
 
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callable spreadsheets as functions
 
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done:
 
indicate whether we're showing formulas or values
comments
save/restore
copy formulas
relative cell reference
bugfix: invalidate *everything* after assigning
complain() should advance the scanner to the end
handle overflow of input field without blowing up
proper error handling
use home() instead of clear_screen()
overflow marker when cell contents don't fit
show current cell's formula on its own line
make formulas start with = (instead of comments starting with #)
top-level plaint line
plaints from non-evaluation functions like file loading & bad keypress
show current cell's plaint, if any, in the plaint line
be able to abort edits with C-g