For now, I'd use something like
Code:
alias hdec {
  var %0 = $($ $+ $0,2), %ifmatchbug = $ifmatch
  $+(!,$iif(!$show,.),hinc) $iif(%0 isnum,$($!1- $+ $calc($0 -1),2) - $+ %0,$1- -1)
  if (%ifmatchbug) { }
}

I can't test it right now, but it should work..
The $ifmatch part is some workaround to deal with the well known (I suppose) $ifmatch bug (which is very old already, it should be fixed too frown), since it uses $iif().

* cold edits his posts 24/7