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Note 8 - Stockholders' Equity - Outstanding Stock Purchase Warrants (Details) - $ / shares
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Outstanding, shares (in shares) 32,751,578 56,442,157
Outstanding, weighted average exercise price (in dollars per share) $ 0.07 $ 0.14
Exercised, shares (in shares) (983,334) 21,884,420
Exercised, weighted average exercise price (in dollars per share) $ 0.05 $ 0.06
Forfeited or expired, shares (in shares) (1,112,001) 1,806,159
Forfeited or expired, weighted average exercise price (in dollars per share) $ 0.57 $ 1
Outstanding, shares (in shares) 30,656,243 32,751,578
Outstanding, weighted average exercise price (in dollars per share) $ 0.05 $ 0.07
Exercisable, shares (in shares) 30,656,243  
Exercisable, weighted average exercise price (in dollars per share) $ 0.05