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: Prevention of etomidate - induced myoclonus : which is superior : Fentanyl , midazolam , or a combination ?
### Output Text:
Prevention of <span class="chemical">etomidate</span> - induced myoclonus : which is superior : <span class="chemical">Fentanyl</span> , <span class="chemical">midazolam</span> , or a combination ? 