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: Finally , the contribution of a primary , thrombotic effect of cocaine has not been excluded .
### Output Text:
Finally , the contribution of a primary , thrombotic effect of <span class="chemical">cocaine</span> has not been excluded . 