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3. So what's the difference between Atop (@) and Compose (&) ?

None at all, for the monads (u@v) and (u&v)

  u&v y ↔ u v y
  u@v y ↔ u v y
But the dyads are different
  x u&v y ↔ (v x) u (v y)
  x u@v y ↔ u x v y

According to the J Dictionary -- &: is equivalent to & except that the ranks of the resulting function are infinite; the relation is similar to that between @: and @