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Note 12 - Summary of Stock Options - Valuation Assumption for Employee Stock Purchase Plan (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Expected term (Year) 6 years 5 years
Average volatility 70.00% 73.00%
Risk-free interest rate 1.62% 2.65%
Dividend yield 0.00% 0.00%
2017 Employee Stock Purchase Plan [Member]    
Expected term (Year) 91 days 91 days
Average volatility 87.00% 72.00%
Risk-free interest rate 2.29% 1.92%
Dividend yield 0.00% 0.00%