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: The ACTIVE - W ( Atrial Fibrillation Clopidogrel Trial with Irbesartan for Prevention of Vascular Events ) study has demonstrated that warfarin is superior to platelet therapy ( clopidogrel plus aspirin ) in the prevention af embolic events .
### Output Text:
The ACTIVE - W ( Atrial Fibrillation <span class="chemical">Clopidogrel</span> Trial with <span class="chemical">Irbesartan</span> for Prevention of Vascular Events ) study has demonstrated that <span class="chemical">warfarin</span> is superior to platelet therapy ( <span class="chemical">clopidogrel</span> plus <span class="chemical">aspirin</span> ) in the prevention af embolic events . 