Changes the BUTFIRST of the
list input to a given value.
list input must be a list and cannot be empty.
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 gets some primitives into infinite loops; unexpected changes to other data structures that share storage with the list being modified; crashes and core dumps if the BUTFIRST of a list is not itself a list; and the permanent loss of memory if a circular structure is released.
MAKE "mylist [1 2 3] .SETBF :mylist [a b] SHOW :mylist[1 a b]