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Note 8 - Fair Value of Financial Instruments (Tables)
12 Months Ended
Dec. 31, 2018
Notes Tables  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]
Warrant liability
       
Beginning balance
  $    
Issuances of warrants
   
1,612,413
 
Warrant exercises
   
(1,893,160
)
Change in fair value
   
280,747
 
Ending balance
  $