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Income taxes - Schedule of effective income tax rate reconciliation (Detail)
12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Income taxes      
Tax expense computed at the federal statutory rate 21.00% 21.00% 21.00%
Change in valuation allowance (33.00%) (25.00%) (22.00%)
Research and development credit benefit 3.00% 1.00% 1.00%
Business combination (2.00%)    
Stock-based compensation 12.00% 3.00%  
Total income tax expense 1.00%