C7: partitioned enclose abbreviated ⍺

[xpqz]

This needs mention of the (new) fact that the left argument to ⊂ can be abbreviated, with missing elements presumed 0, and maybe an example of converting begin-points to mask:

      ⍸⍣¯1⊢1 5 10
1 0 0 0 1 0 0 0 0 1
      (⍸⍣¯1⊢1 5 10)⊂⎕A
┌→───────────────────────────────────┐
│ ┌→───┐ ┌→────┐ ┌→────────────────┐ │
│ │ABCD│ │EFGHI│ │JKLMNOPQRSTUVWXYZ│ │
│ └────┘ └─────┘ └─────────────────┘ │
└∊───────────────────────────────────┘

[xpqz]

Fixed in 55310ce