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Note 15 - Concentration (Details Textual) - USD ($)
3 Months Ended 6 Months Ended
Oct. 31, 2018
Oct. 31, 2017
Oct. 31, 2018
Oct. 31, 2017
Revenue from Contract with Customer, Including Assessed Tax $ 9,051,000 $ 8,989,000 $ 18,022,000 $ 17,903,000
Customer Concentration Risk [Member] | Sales Revenue, Net [Member] | One Single Customer [Member]        
Concentration Risk, Percentage 19.20%      
Revenue from Contract with Customer, Including Assessed Tax $ 18,022,000