In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Carbamazepine was switched to its 10 - keto analogue oxcarbazepine among six difficult - to - treat schizophrenic or organic psychotic patients using concomitantly haloperidol , chlorpromazine or clozapine .
### Output Text:
<span class="chemical">Carbamazepine</span> was switched to its 10 - keto analogue <span class="chemical">oxcarbazepine</span> among six difficult - to - treat schizophrenic or organic psychotic patients using concomitantly <span class="chemical">haloperidol</span> , <span class="chemical">chlorpromazine</span> or <span class="chemical">clozapine</span> . 