The Atop (⍤) operator

The Atop operator is new way to write 2-trains.

A 2-train is evaluated like a 3-train with the central function's left argument removed.

If passed only one argument, an atop (g h) or g⍤h is identical to g∘h.

• ⊃⍤⌽ picks element ⍺ from ⍵'s first row, or the last if called monadically.
• Use f⍤⊣ or f⍤⊢ to apply a function f to only the left or right argument within a train.
• Write ⊢⍤/ for Compress if plain / is treated as an operator.