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: Extracorporeal lithotripsy successfully removed a renal calculus in one patient and surgery removed a staghorn calculus in another , permitting continued treatment .
### Output Text:
Extracorporeal lithotripsy successfully removed a renal calculus in one patient and surgery removed a staghorn calculus in another , permitting continued treatment . 