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: Long - term propranolol therapy in pregnancy : maternal and fetal outcome .
### Output Text:
Long - term <span class="chemical">propranolol</span> therapy in pregnancy : maternal and fetal outcome . 