.SETFIRST list value

	command.  Changes the first member 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 .SETFIRST
	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; and
	the loss of memory if a circular structure is released.

