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: To directly examine the importance of growth to Warfarin - induced artery calcification in animals of the same age , 20 - day - old rats were fed for 2 weeks either an ad libitum diet or a 6 - g / d restricted diet that maintains weight but prevents growth .
### Output Text:
To directly examine the importance of growth to <span class="chemical">Warfarin</span> - induced artery calcification in animals of the same age , 20 - day - old rats were fed for 2 weeks either an ad libitum diet or a 6 - g / d restricted diet that maintains weight but prevents growth . 