A mite of pedantry: the J train (a b c d) is a hook with a fork on the right, and so dyadically acts like

a(x, c(b(y), d(y)))

Roger himself has dismissed [0] hooks as an unfortunate result of J4's myriad train rules, made in the name of tacitable everything, which I lament because for some reason, tacit programming is just so much more satisfying than normally solving the problem.

[0]: http://www.jsoftware.com/jwiki/Essays/Hook%20Conjunction%3F