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: There was no difference in the maximum double product ( p = 0 . 53 ) or ST depression ( p = 0 . 63 ) with either form of stress .
### Output Text:
There was no difference in the maximum double product ( p = 0 . 53 ) or ST depression ( p = 0 . 63 ) with either form of stress . 