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: CONCLUSION : Among non - users of aspirin , naproxen seemed to carry the highest risk for AMI / GI bleeding .
### Output Text:
CONCLUSION : Among non - users of <span class="chemical">aspirin</span> , <span class="chemical">naproxen</span> seemed to carry the highest risk for AMI / GI bleeding . 