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: The authors present three patients with de novo absence epilepsy after administration of carbamazepine and vigabatrin .
### Output Text:
The authors present three patients with de novo absence epilepsy after administration of <span class="chemical">carbamazepine</span> and <span class="chemical">vigabatrin</span> . 