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: In the first case a caesarean section was done one week after replacement of warfarin with heparin .
### Output Text:
In the first case a caesarean section was done one week after replacement of <span class="chemical">warfarin</span> with <span class="chemical">heparin</span> . 