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: Carbamazepine ( CBZ ) , a commonly used AED , has been implicated in some clinical studies .
### Output Text:
<span class="chemical">Carbamazepine</span> ( <span class="chemical">CBZ</span> ) , a commonly used AED , has been implicated in some clinical studies . 