todo:
1) autobackup of things typed in.
	idea: each cell change output to a stdio open file
		in the save format, fflush() every so often...
		(diffs w/r to the original file)
2) make sure ISVALID should <not> be used in place of checkbounds
	in interp.c
3) hide range
4) block moving into range
5) chain cells w/ equations into a linked list or dependency tree
	-have a top level eval, eval and UPDATE all lower nodes
6) an option to go into a ^R like <mode>
	++data entry fields (highlight entry cells)....
	++only allow entry in these cells....
7) don't redraw the whole screen all the time 
	(only cells that change, (in addition to what is in 'fixed #9'))
	(Note: there was no #9 when I took this over, and I've renumbered
	things since then, so I'm not sure what this is referring to - CRM)
8) add uemacs keybinding stuff
9) add uemacs macro language
10) add uemacs command completion
11) add undo
12) rewrite update() function in screen.c from scratch in a more sane manner
	(the current one is buggy and a headache to debug)
13) add a command line option (possibly -g) to use sc as a sort of grep for
	formatted or unformatted data in a file or list of files which will
	output the whole row in which a given number or string is found,
	formatted as with the write command.