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: AIM : Widespread use of mannitol to reduce brain edema and lower elevated ICP in brain tumor patients continues to be afflicted by the so - called rebound phenomenon .
### Output Text:
AIM : Widespread use of <span class="chemical">mannitol</span> to reduce brain edema and lower elevated ICP in brain tumor patients continues to be afflicted by the so - called rebound phenomenon . 