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: Although indomethacin has been implicated as a possible cause of aplastic anemia on the basis of a few clinical observations , its role has not been definitely established .
### Output Text:
Although <span class="chemical">indomethacin</span> has been implicated as a possible cause of aplastic anemia on the basis of a few clinical observations , its role has not been definitely established . 