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: MAIN RESULTS : The incidence of myoclonus was significantly lower in the remifentanil group ( 6 . 7 % ) than in the placebo group ( 70 % ) ( P < 0 . 001 ) .
### Output Text:
MAIN RESULTS : The incidence of myoclonus was significantly lower in the <span class="chemical">remifentanil</span> group ( 6 . 7 % ) than in the placebo group ( 70 % ) ( P < 0 . 001 ) . 