(datatype binary

if (element? X [0 1])
_____________
X : zero-or-one;

X : zero-or-one;
________________
[X] : binary;

X : zero-or-one; Y : binary;
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[X | Y] : binary;

X : zero-or-one, [Y | Z] : binary >> P;
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[X Y | Z] : binary >> P;)

(define complement
   {binary --> binary}
   [0] -> [1] 
   [1] -> [0]      
   [1 N | X] -> [0 | (complement [N | X])] 
   [0 N | X] -> [1 | (complement [N | X])])