[
  { a => 1 },
  { a => 1, b => 2 },
  { a => 3, b => 3, c => 7 },
  { a => 5, b => 6, c => 8 },
  { a => 5, c => 8, d => 9 },
]
{
  a => [1, 1, 3, 5, 5],
  b => [0, 2, 3, 6, 0],
  c => [0, 0, 7, 8, 8],
  d => [0, 0, 0, 0, 9],
}