. — Inner productX f.g Y pairs items with g, then reduces with f. It contracts the last axis of X with the first of Y.
1 2 3+.×4 5 6 ⍝ 32
(2 2⍴1 0 0 1)+.×2 2⍴⍳4 ⍝ 2 2⍴⍳4
Singleton contraction axes extend. Empty contractions use the reduction identity. g⌝ is outer product.