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: CONCLUSIONS : We conclude that pretreatment with fentanyl or combination of fentanyl and midazolam was effective in preventing etomidate - induced myoclonus .
### Output Text:
CONCLUSIONS : We conclude that pretreatment with <span class="chemical">fentanyl</span> or combination of <span class="chemical">fentanyl</span> and <span class="chemical">midazolam</span> was effective in preventing <span class="chemical">etomidate</span> - induced myoclonus . 