dsetfirst
.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.