BUTFIRSTS list
BFS list

	outputs a list containing the BUTFIRST of each member of the input
	list.  It is an error if any member of the input list is empty or an
	array.  (The input itself may be empty, in which case the output is
	also empty.)  This could be written as

		to butfirsts :list
		output map "butfirst :list
		end

	but is provided as a primitive in order to speed up the iteration
	tools MAP, MAP.SE, and FOREACH.

