TALLY (_) #y is 1 if y is an atom, and is otherwise the number of items in y.
COPY (1 _) If the arguments have an equal number of items, then x#y copies +/x items from y, with i{x repetitions of item i{y; if one is an atom, it is repeated to make the item count of the arguments equal; if both are atoms they are treated as one-element lists.