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: This led us to hypothesize that a metabolite of cocaine may be responsible for some of those delayed sequelae .
### Output Text:
This led us to hypothesize that a metabolite of <span class="chemical">cocaine</span> may be responsible for some of those delayed sequelae . 