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Note 9 - Stockholders' Equity - Summary of Information Concerning Outstanding and Exercisable Stock Options (Details)
Jun. 30, 2019
$ / shares
shares
Number of outstanding (in shares) 3,325,750
Number of exercisable (in shares) 3,197,650
Range 1 [Member]  
Number of outstanding (in shares) 770,000
Range of exercise prices per share (in dollars per share) | $ / shares $ 0.42
Number of exercisable (in shares) 770,000
Range 2 [Member]  
Number of outstanding (in shares) 567,750
Range of exercise prices per share (in dollars per share) | $ / shares $ 0.42
Number of exercisable (in shares) 439,650
Range 3 [Member]  
Number of outstanding (in shares) 713,000
Range of exercise prices per share (in dollars per share) | $ / shares $ 0.42
Number of exercisable (in shares) 713,000
Range 4 [Member]  
Number of outstanding (in shares) 1,275,000
Range of exercise prices per share (in dollars per share) | $ / shares $ 0.50
Number of exercisable (in shares) 1,275,000