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: Here we report of two adult patients with a long history of epilepsy treated with PB who died suddenly : one as consequence of cardiac arrest , the other of acute bronchopneumonia .
### Output Text:
Here we report of two adult patients with a long history of epilepsy treated with <span class="chemical">PB</span> who died suddenly : one as consequence of cardiac arrest , the other of acute bronchopneumonia . 