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: We propose that naturally occurring compounds such as flavonoids , which are present in fruit juices , may increase the potency of warfarin by competing for the enzymes that normally inactivate warfarin .
### Output Text:
We propose that naturally occurring compounds such as <span class="chemical">flavonoids</span> , which are present in fruit juices , may increase the potency of <span class="chemical">warfarin</span> by competing for the enzymes that normally inactivate <span class="chemical">warfarin</span> . 