sub import {
   shift;
   return unless @_;
   die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant';
   overload::constant qr => sub {
      my $s= shift;
      $s =~ s/(<!\\)\x{ab}/(?:/g && $s =~ s/(<!\\)\x{bb}/)/g;
      return eval qr/$s/;
      };
   }