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: We report a case in which myocardial infarction coincided with the introduction of captopril and the withdrawal of verapamil in a previously asymptomatic woman with severe hypertension .
### Output Text:
We report a case in which myocardial infarction coincided with the introduction of <span class="chemical">captopril</span> and the withdrawal of <span class="chemical">verapamil</span> in a previously asymptomatic woman with severe hypertension . 