The code itself looks good, though, so I'll approve this once I understand the move flopping.

The flopping outside of +clad happens twice: once in +emil, again in +abet. It looks like the +clad interpreter is just emitting moves in the order it expects them to be run.

Yes, the interpreter produces a list of moves in "natural order". A list of moves is in natural order if we expect the head of the list to be the first move evaluated.

It's common in cores to internally keep moves in reverse order and flop it at the end because that makes it easier to add a single event to the end of the list. If you're doing that, you should be very careful and should perhaps document it. But if we see something accept or produce a list of moves, we assume that it's in natural order.

If you're storing moves in natural order (uncommon in cores), then +emit (which should add a move to be evaluated after all the previously-emitted moves) is =. moves (flop [new-move (flop moves)]). But +emil (which should emit a list of moves in natural order after previous moves) is simple: =. moves (weld moves new-moves).

If you're storing moves in reverse order, as in +de, then +emit is very simple: =. reverse-moves [new-move reverse-moves]. +emil is slightly more complex, and I think there's a couple places where we get this wrong: =. reverse-moves (weld (flop new-moves) reverse-moves).

In both cases, you could define +emil in terms of +emit:

|-  ^-  +>.^$
?~  moves
  +>.^$
=.  +>.^$  (emit i.new-moves)
$(moves t.moves)

We usually store moves in reverse order, but this should be entirely abstracted away by +emit and +emil. Unless you're writing those functions themselves, any time you see a list of moves, you should expect it to be in natural order.

Note: this is targeted against #1169 because I didn't want to push new features directly to #1169 because it's already been reviewed. I also don't want to interfere with #1169 hitting master -- that shouldn't be blocked on this PR.

Feel free to merge this one into #1169, given the fellas' reviews -- I can merge the whole clay & clad goodness into master tomorrow. Or since you know I plan on doing it anyway, you can. 😄

Will do so once @belisarius222 is satisfied with the move order.