class Array::ArrayNotRectangular

group_by_positions: creates a nested hash from an array, like group_by

but with many positions at a time

-> arbitray number of positions, example 'myarray.group_by_positions(0,1,2,3)'

-> array

nested hash ->

{key0=>{key1=>{key2=>{key3=>[ array remaining array elemnts]}}},
 key0=>{key1=>{key2=>{key3=>[ array remaining array elemnts]}}},
 …
} ->