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: Among users of aspirin , they were : rofecoxib 1 . 73 ( 1 . 52 , 1 . 98 ) , celecoxib 1 . 34 ( 1 . 19 , 1 . 52 ) , ibuprofen 1 . 51 ( 0 . 95 , 2 . 41 ) , diclofenac 1 . 69 ( 1 . 35 , 2 . 10 ) , naproxen 1 . 35 ( 0 . 97 , 1 . 88 ) and acetaminophen 1 . 29 ( 1 . 17 , 1 . 42 ) .
### Output Text:
Among users of <span class="chemical">aspirin</span> , they were : <span class="chemical">rofecoxib</span> 1 . 73 ( 1 . 52 , 1 . 98 ) , <span class="chemical">celecoxib</span> 1 . 34 ( 1 . 19 , 1 . 52 ) , <span class="chemical">ibuprofen</span> 1 . 51 ( 0 . 95 , 2 . 41 ) , <span class="chemical">diclofenac</span> 1 . 69 ( 1 . 35 , 2 . 10 ) , <span class="chemical">naproxen</span> 1 . 35 ( 0 . 97 , 1 . 88 ) and <span class="chemical">acetaminophen</span> 1 . 29 ( 1 . 17 , 1 . 42 ) . 