space is not left for empty diagram when using e.g. hcat' with {sep = ... }

[fryguybob]

(Imported from http://code.google.com/p/diagrams/issues/detail?id=32. Original issue from byor...@gmail.com on May 24, 2011, 10:09:35 PM UTC)

I was surprised today to find that

hcat' with {sep = 1} [square, mempty, square]

and

hcat' with {sep = 1} [square, square]

produce the same output. However, the first ought to produce two squares with two units of space between them: one unit between the first square and mempty, and one unit between mempty and the second square.

[fryguybob]

(Imported. Original comment by byor...@gmail.com on June 8, 2011, 06:59:25 PM UTC)

The problem was that cat' worked by (among other things) calling 'align' on some of the intermediate results of the composition, which doesn't work correctly if you have just positioned 'mempty' a certain distance away from another diagram -- composing with 'mempty' doesn't affect the bounds, so the only information we have left about where the next diagram is supposed to go is the position of the local origin. Basically the fix was to carefully orchestrate things so we only ever call 'boundary' and 'align' on single input diagrams to get them set up properly and then during the actual composition we just move local origins around.