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: Crossover comparison of efficacy and preference for rizatriptan 10 mg versus ergotamine / caffeine in migraine .
### Output Text:
Crossover comparison of efficacy and preference for <span class="chemical">rizatriptan</span> 10 mg versus <span class="chemical">ergotamine</span> / <span class="chemical">caffeine</span> in migraine . 