Classic Edition: the symbol ⊆ (Left Shoe Underbar) is not available in Classic Edition, and Nest is instead represented by ⎕U2286.
If Y is simple, R is a scalar array whose item is the array Y. If Y is a simple scalar or is already nested, R is Y unchanged.
⊆1 2 3 ┌─────┐ │1 2 3│ └─────┘ ⊆ 1 (1 2 3) ┌─┬─────┐ │1│1 2 3│ └─┴─────┘ ⊆'Dyalog' ┌──────┐ │Dyalog│ └──────┘ ⊆'Dyalog' 'APL' ┌──────┬───┐ │Dyalog│APL│ └──────┴───┘