⊆ — Nest / Partition⊆Y encloses a simple non-scalar array; scalars and already-nested arrays stay unchanged.
⊆1 2 ⍝ ⊂1 2
⊆1 ⍝ 1
N⊆Y partitions along the last axis: zero omits, a rise in the mark starts a partition.
1 1 0 1 1⊆'abcde' ⍝ 'ab' 'de'
Marks are nonnegative integers. Scalar/singleton marks extend. [K] selects the axis. Unlike ⊂, positive marks need not start a partition at every position.