.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; Logo
	crashes and coredumps if the butfirst of a list is not itself a list;
	and the loss of memory if a circular structure is released.

