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Note I - Stock-based Compensation (Tables)
 6 Months Ended
Oct. 31, 2018
Notes Tables  
Stock Option Plan Comparison [Table Text Block]
    Restated Option Plan
Minimum exercise price as a percentage of fair market value at date of grant    
100
 %
Last expiration date for outstanding options    
May 8, 2028
  
Shares available for grant at October 31, 2018    
298,500
  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
    Six Months Ended
October 31,
    2018   2017
Expected term (years)    
5.5
      
5.5
  
Risk-free interest rate    
2.79
 %    
1.81
 %
Volatility    
36
 %    
36
 %
Dividend yield    
-
      
-
  
Schedule of Share-based Compensation, Stock Options, Exercises [Table Text Block]
    Six Months Ended
October 31,
(Dollars in thousands)   2018   2017
Options exercised    
220,750
      
113,000
  
Cash received from option exercises   $
4,294
     $
822
  
Intrinsic value of options exercised   $
8,317
     $
2,998