dsetbf
.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.