.SETBF list value

	command.  Changes the butfirst of "list" to be "value".

	WARNING: Primitives whose names start with a period are DANGEROUS.
	Their use by non-experts is not recommended.  The use of .SETBF can
	lead to circular list structures, which will get some Logo primitives
	into infinite loops; unexpected changes to other data structures that
	share storage with the list being modified; or to Logo crashes and
	coredumps if the butfirst of a list is not itself a list.