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 : Four patients with corneal ulcers associated with crack cocaine use were reviewed .
### Output Text:
RESULTS : Four patients with corneal ulcers associated with <span class="chemical">crack cocaine</span> use were reviewed . 