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: In contrast , no artery calcification could be detected in 10 - month - old adult rats even after 4 weeks of Warfarin treatment .
### Output Text:
In contrast , no artery calcification could be detected in 10 - month - old adult rats even after 4 weeks of <span class="chemical">Warfarin</span> treatment . 