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: A single case report suggests that yohimbine may be used to treat the sexual side effects of clomipramine .
### Output Text:
A single case report suggests that <span class="chemical">yohimbine</span> may be used to treat the sexual side effects of <span class="chemical">clomipramine</span> . 