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: The [ verapamil ] o that arrested atrial beating ( AC ) was also potentiated with the order LNa = LNa + LCa = LNa + HCa = LCa > HCa = N .
### Output Text:
The [ <span class="chemical">verapamil</span> ] o that arrested atrial beating ( AC ) was also potentiated with the order LNa = LNa + LCa = LNa + HCa = LCa > HCa = N . 