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Derivatives - Assumptions used to determine fair value of derivative warrant liability (Details) - Fair value of derivative warrant liability
12 Months Ended
Apr. 30, 2021
Risk-free interest rate, minimum 0.08%
Risk-free interest rate, maximum 0.61%
Expected dividend yield
Expected term, minimum 1 year 6 months 15 days
Expected term, maximum 5 years
Expected volatility, minimum 86.74%
Expected volatility, maximum 110.56%