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: Alopecia , paresthesias , and arthralgias / myalgias have occurred frequently , but with one exception ( a grade 3 myalgia ) they have been grade 1 or 2 .
### Output Text:
Alopecia , paresthesias , and arthralgias / myalgias have occurred frequently , but with one exception ( a grade 3 myalgia ) they have been grade 1 or 2 . 