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: Of 18 consecutive referrals of children with resistant typical absences only , eight were erroneously treated with carbamazepine either as monotherapy or as an add - on .
### Output Text:
Of 18 consecutive referrals of children with resistant typical absences only , eight were erroneously treated with <span class="chemical">carbamazepine</span> either as monotherapy or as an add - on . 