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: OBJECTIVES : To determine the incidence of major hemorrhage and stroke in people aged 76 and older with atrial fibrillation on adjusted - dose warfarin who had been recently been admitted to hospital .
### Output Text:
OBJECTIVES : To determine the incidence of major hemorrhage and stroke in people aged 76 and older with atrial fibrillation on adjusted - dose <span class="chemical">warfarin</span> who had been recently been admitted to hospital . 