CDE = pointer to previous entry
BHL = pointer to current entry
OUT, if call successful:
Fc = 0
CDE = pointer to old current entry (ie. BHL(in))
BHL = pointer to next entry
Fz = 1, if BHL(out) = 0.
OUT, if call failed:
Fc = 1
If BHL(in) = 0; BHL(in) and CDE(in) are then preserved.
A = error code:
RC_EOF ($09), attempt to step beyond start or end of list
Registers changed after return:
For more details, read Linked lists.
GN_Xdl, delete an entry from a linked list
GN_Xin, insert an entry into a linked list