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: Encephalopathy induced by levetiracetam added to valproate .
### Output Text:
Encephalopathy induced by <span class="chemical">levetiracetam</span> added to <span class="chemical">valproate</span> . 