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: All 4 patients presented with skin eruptions that developed after receiving warfarin for several years .
### Output Text:
All 4 patients presented with skin eruptions that developed after receiving <span class="chemical">warfarin</span> for several years . 