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{
  head1 => { val1 => 2, val2 => 1, val3 => 1, val6 => 1 },
  head2 => { val2 => 1, val4 => 2, val7 => 2 },
  head3 => { val3 => 2, val5 => 3 },
}

{
  val1 => [-1],   # val1 occurs more than once in col 1
...
  val6 => [1],
  val7 => [-2], #val7 is mentioned at least twice in col2
}