Consolidated Condensed Balance Sheets (Current Period Unaudited) (Parentheticals) - USD ($) $ in Thousands	Jan. 31, 2021	Apr. 30, 2020
Short term investments	$ 15,286	$ 2,115
Allowance for doubtful accounts	$ 37	$ 38
Common stock, par value (in dollars per share)	$ 0.10	$ 0.10
Common stock, shares authorized (in shares)	30,000,000	30,000,000
Common stock, shares issued (in shares)	10,000,000	10,000,000
Treasury stock, shares (in shares)	420,360	383,279