Since day one, all APL systems have always used 6 spaces as a prompt for user input.
That's fine, but now it could be a little more flexible.
Suppose the user preferences for the session manager allowed the prompt to be set to a short character vector.
And suppose you set it to '⋄ ' . That's a diamond and 5 spaces.
The behavior of the session would be unchanged, but all user inputs would be labeled clearly and distinctly.