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Note 9 - Preferred Stock and Common Stock Warrants (Tables)
3 Months Ended
Mar. 31, 2020
Notes Tables  
Fair Value Measurement Inputs and Valuation Techniques [Table Text Block]
   
March 31,
   
December 31,
 
   
2020
   
2019
 
Stock price
  $
1.88
    $
6.14
 
Exercise price
  $
0.01
    $
0.01
 
Risk-free rate
   
0.2
%    
1.6
%
Volatility
   
48.8
%    
27.6
%
Term (years)
   
1.6
     
1.8