The Rank (⍤) operator

Rank is a dyadic operator, so it always takes two operands:

⊂⍤2

Much like ⍨ guarantees that its operand is called with two arguments (dyadically), ⍤ guarantees that its operand is called only on arrays whose rank is less than or equal to the specified rank.

If an argument array's rank is too large, Rank maps over cells of the array.