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: Epicardial mapping suggested that verapamil promoted AF by increasing the number of simultaneous wavefronts reflected by separate zones of reactivation in each cycle .
### Output Text:
Epicardial mapping suggested that <span class="chemical">verapamil</span> promoted AF by increasing the number of simultaneous wavefronts reflected by separate zones of reactivation in each cycle . 