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: Absence of acute cerebral vasoconstriction after cocaine - associated subarachnoid hemorrhage .
### Output Text:
Absence of acute cerebral vasoconstriction after <span class="chemical">cocaine</span> - associated subarachnoid hemorrhage . 