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### Input Text: METHODS : We performed a systematic review of published and unpublished data from cohorts with stroke or TIA to compare the presence of MB in : ( 1 ) antithrombotic users vs nonantithrombotic users with ICH ; ( 2 ) antithrombotic users vs nonusers with IS / TIA ; and ( 3 ) ICH vs ischemic events stratified by antithrombotic use .
### Output Text:
METHODS : We performed a systematic review of published and unpublished data from cohorts with stroke or TIA to compare the presence of MB in : ( 1 ) antithrombotic users vs nonantithrombotic users with ICH ; ( 2 ) antithrombotic users vs nonusers with IS / TIA ; and ( 3 ) ICH vs ischemic events stratified by antithrombotic use . 