This is an experimental spreadsheet to test FrTime and its GUI
development capabilities.  It is interesting because FrTime is used to
implement the spreadsheet and as the language for cell formulas.

Usage: 

To run the spreadsheet, open spread.ss in DrScheme, set the language level
to (module ...), and execute.

Select a cell by clicking the mouse or moving with the arrow
keys.  Press enter to focus the text entry field, where you can enter
a FrTime expression.  This includes purely functional Scheme and many
common primitives.  In a cell, you can refer to another cell by name.
Entering the formula (+ a5 c7), including the parentheses, makes the
value of the selected cell the sum of cells a5 and c7.

It is also possible to refer to sequences of adjacent cells, which
results in a list.  For example, a1:5 returns a list containing the
values of the cells a1 through a5.

The spreadsheet can load and save files.  For several of examples of
spreadsheet formulas, including the use of behaviors, cell sequences,
and absolute cell references, open demos.sheet.

Known Bugs:

- Initial evaluation of a cell formula is super slow.
- Whole-screen redraw, as when scrolling or resizing, is super slow.
- Errors arising during re-evaluation (not during initial evaluation) go
  to the DrScheme interactions window instead of propagating to the cell.
- Does not work on MacOS