Leading axis theory

In modern APL, we consider each high-rank array to be a list of major cells. The number of major cells is the first entry of the shape, also given by Tally (≢), and the shape of each major cell is the rest of the shape.

      ⊢d ← 2 3 4 ∘.∨ 2 3 6 7  ⍝ ∨ is the Greatest Common Divisor
2 1 2 1
1 3 3 1
2 1 2 1
      ≢d  ⍝ Number of major cells
3
      ∪d  ⍝ Unique major cells
2 1 2 1
1 3 3 1

Split (↓) doesn't follow the leading axis rule because it predates it!