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: Overall , in high - risk patients , warfarin is superior to aspirin in preventing strokes , with a relative risk reduction of 36 % .
### Output Text:
Overall , in high - risk patients , <span class="chemical">warfarin</span> is superior to <span class="chemical">aspirin</span> in preventing strokes , with a relative risk reduction of 36 % . 