NOTES

 a&b

    a       b
   ___     ___
    |       |
    |       |
    |       |
    |       |
    1       1
     \     /
      \   /
        2

 a&a

    a
   ___
    |
    |
    |
    |
    2
    |
    |
    2


Each channel has a map from (joinid -> inputhandler)
Each inputhandler gets given the current channel length
	and sets its state accordingly
If its state changes, it signals the change to its join
If the join is complete, it enables itself


When a particular join is chosen, it disables itself
	and for each channel in the join removes the join's
	entry from the channel's joinid map

The enabled branches should be a map (joinid -> join)

Each join has a rate and a continuation

Rates:

  1'		k[a]		mol/L		* L * (1/s)/mol == 1/s
  2'		k[a][b]		mol^2/L^2	* L * (1/s)/(mol^2/L) == 1/s
  3'		k[a][b][c]	mol^3/L^3	* L * (1/s)/(mol^3/L^2) == 1/s

I'm unsure as to the physical interpretation of second- and third-order rates.