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 patients with atrial fibrillation digoxin is beneficial for ventricular rate control .
### Output Text:
In patients with atrial fibrillation <span class="chemical">digoxin</span> is beneficial for ventricular rate control . 