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: A significant proportion of warfarin - related cerebral haemorrhages might have been prevented if greater caution had been taken when prescribing drugs known to interact with warfarin .
### Output Text:
A significant proportion of <span class="chemical">warfarin</span> - related cerebral haemorrhages might have been prevented if greater caution had been taken when prescribing drugs known to interact with <span class="chemical">warfarin</span> . 