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

##</code><code>##

{
  val1 => [-1],   # val1 occurs more than once in col 1
  val2 => [2, 1], # val2 occurs once in col 1 and once in col 2
  val3 => [-3, 1], #val3 occurs more than once in col 3
                   #but only one time in col1
  val4 => [-2],
  val5 => [-3],
  val6 => [1],
  val7 => [-2], #val7 is mentioned at least twice in col2
}