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Note 8 - Share-based Compensation - Unvested Balance of Options (Details) - $ / shares
 3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Unvested, number of options (in shares) 2,008,000  
Unvested, weighted average exercise price per share (in dollars per share) $ 4.72  
Unvested, weighted average remaining contractual term (Year) 9 years 36 days 9 years 36 days
Granted, number of options (in shares) 478,000  
Granted, weighted average exercise price per share (in dollars per share) $ 2.48  
Granted, weighted average remaining contractual term (Year) 9 years 292 days  
Vested, number of options (in shares) (1,004,000)  
Vested, weighted average exercise price per share (in dollars per share) $ 4.72  
Vested, weighted average remaining contractual term (Year) 8 years 292 days  
Unvested, number of options (in shares) 1,482,000 2,008,000
Unvested, weighted average exercise price per share (in dollars per share) $ 4 $ 4.72