REPL: allow `#-n` to be used, which will count backgrounds.
REPL: allow `#-n` to be used, which will count backgrounds.

Example:

	> 2
	#1 = 2
	> prime(#-1)
	#2 = 3
	> prime(#-1)
	#3 = 5
	> prime(#-1)
	#4 = 11
	>

trizen/sidef,trizen/sidef,trizen/sidef,trizen/sidef