Function face

    ∇ Z←face N;Pos;Axis;HL;IDX
[1]  ⍝
[2]  ⍝⍝ return the 3D cube numbers and the 3D axis for 2D face N
[3]  ⍝
[4]  IDX←(1 2 3) (1 2 3) (1 2 3)
[5]  IDX[Axis←(3 1 2 1 2 3)[N]]←1+2×N≥4
[6]  Z←(N mirror ((⌽IDX)⌷3 3 3⍴⍳27)) Axis
    ∇