C7: partitioned enclose head/tail example

[xpqz]

It'd also be nice to show the "head and tail" trick:

      1 1⊂'Hello'
┌→───────────┐
│ ┌→┐ ┌→───┐ │
│ │H│ │ello│ │
│ └─┘ └────┘ │
└∊───────────┘

[xpqz]

Fixed in 55310ce