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: PARTICIPANTS : Two hundred thirty - five patients aged 76 and older admitted to a major healthcare network between July 1 , 2001 , and June 30 , 2002 , with atrial fibrillation on warfarin were enrolled .
### Output Text:
PARTICIPANTS : Two hundred thirty - five patients aged 76 and older admitted to a major healthcare network between July 1 , 2001 , and June 30 , 2002 , with atrial fibrillation on <span class="chemical">warfarin</span> were enrolled . 