x (+⌿ ×⍤¯1)⍤1 2 ⊢y

   x←?3 4⍴100
   y←?4 5⍴100

   (x+.×y) ≡ x (+⌿ ×⍤¯1)⍤1 2 ⊢y
1

row-at-a-time vs. row-by-column

•  cache effects
•  zeros in left argument
•  boolean left argument