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: The most severe adverse reactions to carbamazepine have been observed in the haemopoietic system , the liver and the cardiovascular system .
### Output Text:
The most severe adverse reactions to <span class="chemical">carbamazepine</span> have been observed in the haemopoietic system , the liver and the cardiovascular system . 