:: samefringe
::
::  plan:  descend / recurse until each side hits a leaf, then compare.
::         If not equal, fail immediately.
::         If equal,
::
::        .                     .
::      /  \                   /  \
::  => 1   .              =>  .    3
::        /  \               / \
::    	2    3               1   2
::
::
::
::
::
|=  [a=* b=*]
^-  @
12
::  ?: ?=(@ a)
::    ?: ?=(@ b)
::      3 ::=(a b)          :: degenerate case: we're comparing leaf X with leaf X
::    4 :: $(a a, b -:b)
::  5 :: $(a -:a, b -:b)