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Note 5 - Equity - Stock Option Activity (Details) - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Outstanding, number of shares (in shares) 5,982,923 4,535,681
Outstanding, weighted average exercise price (in dollars per share) $ 2.66 $ 3.31
Options granted, number of shares (in shares) 1,165,325 1,744,300
Options granted, weighted average exercise price (in dollars per share) $ 2 $ 1.48
Options exercised, number of shares (in shares)
Options exercised, weighted average exercise price (in dollars per share)
Options expired, number of shares (in shares) (297,058)
Options expired, weighted average exercise price (in dollars per share)
Outstanding, number of shares (in shares) 7,148,248 5,982,923
Outstanding, weighted average exercise price (in dollars per share) $ 2.55 $ 2.66