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: This supports and extends previous evidence for a mechanism by which enflurane and / or isoflurane could produce a hypersensitivity condition similar to that of halothane hepatitis either alone or subsequent to halothane administration .
### Output Text:
This supports and extends previous evidence for a mechanism by which <span class="chemical">enflurane</span> and / or <span class="chemical">isoflurane</span> could produce a hypersensitivity condition similar to that of <span class="chemical">halothane</span> hepatitis either alone or subsequent to <span class="chemical">halothane</span> administration . 