stack roll operator

n j roll. reorder the top n elements of stack. shift all n elements by j time.

stack roll operation

stack before	operation	stack after	comment
1 2 3 4 5	2 1 roll	1 2 3 5 4	same as exch
1 2 3 4 5	3 1 roll	1 2 5 3 4	put the top element and insert at some position
1 2 3 4 5	count 1 roll	5 1 2 3 4	put the top element at the bottom
1 2 3 4 5	4 -1 roll	1 5 2 3 4	take an element from middle and put at top position