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Note 4 - Accounts Receivable (Tables)
 3 Months Ended
Mar. 31, 2019
Notes Tables  
Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block]
   
March 31,
    
December 31,
  
   
201
9
    
201
8
  
Accounts receivable - current
   $
754,204
     $
1,587,817
  
Accounts receivable - non current
    
1,720,000
      
1,720,000
  
     
2,474,204
      
3,307,817
  
Allowance for doubtful accounts - current
    
(13,785
)
    
(13,785
)
Allowance for doubtful accounts - non current
    
(1,720,000
)
    
(1,720,000
)
     
(1,733,785
)
    
(1,733,785
)
                 
Accounts receivable, net of allowances for doubtful accounts
   $
740,419
     $
1,574,032