FIRSTS list

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

		to firsts :list
		output map "first :list
		end

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