.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, and to unexpected changes to other data
	structures that share storage with the list being modified.