> > for a start \exp_after:wN should normally not be needed except on very
 > > low-level programming (which should largely only happen in the kernel). For
 > > the rest the l3expan concept really provides a much more powerful and
 > > consistent interface.
 > In my case I need it indeed as a replacement of a high-level function:
 > namely, a variant of \tl_map_inline that looks at three groups at a
 > time. This is trivial with a macro that takes three undelimited and one
 > "rest" argument, so perhaps l3tl could have \tl_map_function_ii,
 > \lt_map_inline_ii and so on. Then I wouldn't need this any more. 

could you provide some code examples of what you are after? quite likely we
miss something that should be provided, on the other hand perhaps not and
there are good ways to do things differently only not well explained