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: DISCUSSION : An adverse interaction between clonidine and verapamil has not been reported previously .
### Output Text:
DISCUSSION : An adverse interaction between <span class="chemical">clonidine</span> and <span class="chemical">verapamil</span> has not been reported previously . 