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Note 6 - Stockholders' Equity and Stock-based Compensation - ESPP Assumptions (Details) - The 2017 Employee Stock Purchase Plan [Member]
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Expected volatility 83.00% 78.00% 70.00%
Dividend yield 0.00% 0.00% 0.00%
Minimum [Member]      
Expected term in years (Year) 6 months 6 months 6 months
Risk-free interest rate 0.60% 0.06% 0.10%
Maximum [Member]      
Expected term in years (Year) 1 year 1 year 1 year
Risk-free interest rate 3.50% 0.10% 1.00%