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: Erythema nodosum associated with all - trans - retinoic acid ( ATRA ) for acute promyelocytic leukemia ( APL ) is very rare .
### Output Text:
Erythema nodosum associated with <span class="chemical">all - trans - retinoic acid</span> ( <span class="chemical">ATRA</span> ) for acute promyelocytic leukemia ( APL ) is very rare . 