(
  "original sets",
  { A => [1, 2, 3], B => [3, 4], C => [1, 3, 4] },
)
(
  "intersection",
  {
    1 => { A => 1, C => 1 },
    2 => { A => 1 },
    3 => { A => 1, B => 1, C => 1 },
    4 => { B => 1, C => 1 },
  },
)
items in more than one set:
    '1' is in 'A' and 'C'
    '3' is in 'A' and 'B' and 'C'
    '4' is in 'B' and 'C'