my $previous=undef;
my $ex=1;
while(<>) {
  chomp();
  @gtf=split(/\s+/, $_);
  if( ($#gtf>=9) &&
      defined($previous) &&
      ($gtf[9] eq $previous) )
  {
    $ex+=1;
  }
  else
  {
    $ex=1;
  }
  $previous=($#gtf>=9)?$gtf[9]:undef;
  $gtf[2]="ex".$ex;
}