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Note 9 - Fair Value of Financial Instruments (Tables)
3 Months Ended
Mar. 31, 2020
Notes Tables  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]
   
Fair Value Measurements of Level 3 liability-classified warrant
 
Warrant liability at December 31, 2019
  $
6,116,882
 
Increase in fair value of warrant liability
   
16,065
 
Exercise of warrants
   
 
Warrant liability at March 31, 2020
  $
6,132,947