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: Ancrod has been used successfully for prophylaxis against development of thrombosis in patients with heparin induced platelet aggregation who require brief reexposure to heparin , but its success in patients who have developed the thrombosis syndrome is not well defined .
### Output Text:
Ancrod has been used successfully for prophylaxis against development of thrombosis in patients with <span class="chemical">heparin</span> induced platelet aggregation who require brief reexposure to <span class="chemical">heparin</span> , but its success in patients who have developed the thrombosis syndrome is not well defined . 