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: The results suggest the existence of residual beneficial clinical aftereffects of consecutive daily applications of low - frequency rTMS on dyskinesias in PD .
### Output Text:
The results suggest the existence of residual beneficial clinical aftereffects of consecutive daily applications of low - frequency rTMS on dyskinesias in PD . 