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: Cardiac arrest in a child with cerebral palsy undergoing sevoflurane induction of anesthesia after preoperative clonidine .
### Output Text:
Cardiac arrest in a child with cerebral palsy undergoing <span class="chemical">sevoflurane</span> induction of anesthesia after preoperative <span class="chemical">clonidine</span> . 