input = 2, 3, 5, 7

output:
2
2 3
2 3 5
2 3 5 7
-------
2 3   7
-------
2   5
2   5 7
-------
2     7
-------
  3
  3 5
  3 5 7
-------
  3   7
-------
    5
    5 7
-------
      7
-------