How many elements in an array?

The total number of elements in an array is called its "bound".

When we take an outer product on two arrays, we multiply their bounds to find the bound of the result.

The bound of an array is the product of its entire shape:

      ×/ 2 3 4 5
120
      ×/ ⍴ (⍳2 3) ∘., (⍳4 5)
120
      ×/ (⍴ ⍳2 3) , (⍴ ⍳4 5)
120
      (×/ ⍴ ⍳2 3) × (×/ ⍴ ⍳4 5)
120