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: Bilateral basal ganglia infarcts after the use of cocaine , without concurrent heroin use , have never been reported .
### Output Text:
Bilateral basal ganglia infarcts after the use of <span class="chemical">cocaine</span> , without concurrent <span class="chemical">heroin</span> use , have never been reported . 