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: People aged over 75 in atrial fibrillation on warfarin : the rate of major hemorrhage and stroke in more than 500 patient - years of follow - up .
### Output Text:
People aged over 75 in atrial fibrillation on <span class="chemical">warfarin</span> : the rate of major hemorrhage and stroke in more than 500 patient - years of follow - up . 