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: METHODS : A double - blind double - armed prospective study comprised 40 patients who had uneventful sutureless phacoemulsification under sub - Tenon ' s local infiltration of 3 mL of plain lignocaine .
### Output Text:
METHODS : A double - blind double - armed prospective study comprised 40 patients who had uneventful sutureless phacoemulsification under sub - Tenon ' s local infiltration of 3 mL of plain <span class="chemical">lignocaine</span> . 