sub crop_binary_insert {
  my ($a, $x) = @_;
  my ($min, $max) = (0, $#{$a});

  return if ($a->[-1] >= $x); # don't bother to insert if we don't have to

  while ($min != $max) {
    my $mid = int( ($min + $max)/2 );
    if ($a->[$mid] > $x) {
      $min = $mid+1;
    } else {
      $max = $mid;
    }
  }
  pop @$a;
  splice @$a, $min, 0, $x;
}