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: Omitting fentanyl reduces nausea and vomiting , without increasing pain , after sevoflurane for day surgery .
### Output Text:
Omitting <span class="chemical">fentanyl</span> reduces nausea and vomiting , without increasing pain , after <span class="chemical">sevoflurane</span> for day surgery . 