question marks in default index_format

[Paul E Condon <pecondon@xxxxxxxxxxxxxxxx>]

I'm puzzled by the default index_format which is, I think:

set index_format="%4C %Z %{%b %d} %-15.15L (%?l?%4l&%4c?) %s"

I don't understand the string inside parentheses. Where can
I find an explanation? Question marks often invoke some sort
of conditional action, but I don't see them in printf docs.

TIA
-- 
Paul E Condon           
pecondon@xxxxxxxxxxxxxxxx