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Note 6 - Common Stock Options (Tables)
9 Months Ended
Sep. 30, 2020
Notes Tables  
Share-based Payment Arrangement, Option, Activity [Table Text Block]
   

Options

   

Exercise Price

 

Weighted-Average
Exercise Price

 

Options outstanding at December 31, 2019

    6,965,730    

$2.71

- $21.99     6.09  

Granted

    214,500     8.89 - 14.77     16.71  

Exercised

    (1,704,905 )   2.71 - 19.09     6.11  

Forfeitures

    (57,042 )   4.26 - 21.80     8.53  

Options outstanding at September 30, 2020

    5,418,283    

$2.71

- $21.99     6.20