Classic Edition: the symbol ⊆ (Left Shoe Underbar) is not available in Classic Edition, and Nest is instead represented by ⎕U2286.

Y may be any array.

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│
└──────┴───┘