As I see it, it is not an analogy between the display of adverbs,
conjunctions, forks, hooks. It's a rule for displaying a function (in the
general sense) with one, two, or three arguments, and more generally
displaying a tree with an even number of branches (odd number of branches
is symmetric in an obvious way).
One argument (current display already symmetric)
── / ─── + +/
── \ ─── / ─── >. >./\
Two arguments (symmetric version)
┌─ +
──┤ +%
└─ %
┌─ ,.
── &. ─┤ ,.&.":
└─ ":
Three arguments (current display already symmetric):
┌─ / ─── +
──┼─ % +/%#
└─ #
I leave as an exercise for the reader :-) how to modify the non-symmetric
http://www.jsoftware.com/jwiki/Essays/Tree_Display and
https://groups.google.com/forum/#!topic/comp.lang.apl/Xxwd971y_lE to make
them symmetric.
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