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: RESULTS : Thirteen patients with positive cocaine toxicology were compared to 26 controls .
### Output Text:
RESULTS : Thirteen patients with positive <span class="chemical">cocaine</span> toxicology were compared to 26 controls . 