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: OBJECTIVE : A case of acute myocarditis associated with the commencement of clozapine is described , highlighting the onset , course and possible contributing factors .
### Output Text:
OBJECTIVE : A case of acute myocarditis associated with the commencement of <span class="chemical">clozapine</span> is described , highlighting the onset , course and possible contributing factors . 