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 : The rate of major hemorrhage was high in this old , frail group , but excluding fatalities , resulted in no long - term sequelae , and the stroke rate on warfarin was low , demonstrating how effective warfarin treatment is .
### Output Text:
CONCLUSION : The rate of major hemorrhage was high in this old , frail group , but excluding fatalities , resulted in no long - term sequelae , and the stroke rate on <span class="chemical">warfarin</span> was low , demonstrating how effective <span class="chemical">warfarin</span> treatment is . 