Planned Group r←x⊆y

This primitive or feature is planned, that is, it doesn't appear in the most recent released implementation of TinyAPL. Details may change before it is added!

x is a vector of naturals, with length equal to the tally of y. r is a nested vector containing major cells of y, in the positions marked by corresponding entries in x. Cells that correspond to a 0 are dropped from r.