I am more familiar with k so will use it as an example. If we were to extend it, the equivalent of some functions in compositor.nl would be something like this:

  compositeSrc:{x} // output first arg
  compositeDst:{y} // output second arg
  compositeInvert:{+(1-x[;0];1-x[;1];1-x[;2];x[;3])}
  // take a column, invert r, g, b columns and transpose

where x[i;0 1 2 3] == ith (r,g,b,a) of x. No need for "∀ (A, B) >>". This is implicit as k/APL work on whole collections. Basic operations are already overloaded. There would need to be a way to define tuples. "x[i;2]" to refer to ith blue value is a bit cryptic. You can define verb "blue x[i]" to do the same but may be some language syntax would help. I can't grasp enough of Nile through the examples I've seen so far but seems doable.