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Note 5 - Equity - Common Stock Purchase Warrant Activity (Details) - $ / shares
 3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Outstanding, number of warrants (in shares) 7,476,056 3,864,552
Outstanding, weighted average exercise price (in dollars per share) $ 1.45 $ 2.36
Issued, number of warrants (in shares) 0 3,611,504
Issued, weighted average exercise price (in dollars per share) $ 0.46
Cancelled or expired, number of warrants (in shares)
Cancelled or expired, weighted average exercise price (in dollars per share)
Exercised, number of warrants (in shares)
Exercised, weighted average exercise price (in dollars per share)
Outstanding, number of warrants (in shares) 7,476,056 7,476,056
Outstanding, weighted average exercise price (in dollars per share) $ 1.45 $ 1.45