⎕IO←0
while mathematicians sometimes will, and business people will usually stick to ⎕IO←1
. Ancient code will use ⎕ML←0
and new will use ⎕ML←1
while medium-aged code, and code that has been ported from or written by people from other APLs will use ⎕ML←3
. I don't think I've ever seen ⎕ML←2
used.