CMC: for (2≤≢⍵)∧1=⍴⍴⍵, if ≢⍵ is odd, extract odd elements and even elements, e.g. for 1 0 2 3 4 return (1 2 4)(0 3), if ≢⍵ is even, the last element belongs to both sequence, e.g. for 1 0 2 3 return (1 2 3)(0 3)
@pVCaecidiosporeadduced ∘ with an array operand is "Currying". It binds the 2 as constant left argument to |, thus deriving a monadic function you could call "parity". Technically speaking, ∘| can stand alone as a monadic operator, but that is probably not what you have in mind.
@pVCaecidiosporeadduced Just asking, cause when I didn't know about the inversion trick, I used to put some twos to be sure I always get the first "valid" one...