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: ST elevation with chest discomfort disappeared since he began taking long - acting diltiazem .
### Output Text:
ST elevation with chest discomfort disappeared since he began taking long - acting <span class="chemical">diltiazem</span> . 