This is implicit composition ('). You can see this from the parse tree:
q)parse "(3#,:)" ' (#;3) ,: In k, the expression ,: corresponds to
enlist. This construct is the same as repetition (*) in Python. See:
Another excellent paper - thanks Stephen. You mention that iterators
iterate through lists and maps. We know that tables are lists, still, it
may be worth mentioning explicitly that implicit iteration works on
tables as well?