A string in a cell which evaluates to a (floating point)
	numeric result. The
	simplest expression is a number; more complicated expressions
	can be entered in Scheme syntax.
	
	The Scheme used in Siag has a few extensions to make it more
	useful in a spreadsheet. These new functions have been added:
	
		- (row) Returns the row number of the
			cell which is being computed.
		
- (col) Returns the column number of
			the cell which is being computed.
		
- (get-cell row col) Returns
			the contents of the cell with the given
			coordinates.
		
- (sum start end) Returns the sum
			of all the cells in the rectangle between start
			and end.