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: Normalizing effects of modafinil on sleep in chronic cocaine users .
### Output Text:
Normalizing effects of <span class="chemical">modafinil</span> on sleep in chronic <span class="chemical">cocaine</span> users . 