X1 and X2: independent fair bits; information unit: bit
flip=0.00 I(X1;Y)=0.000000 I(X2;Y)=0.000000 I(pair;Y)=1.000000
flip=0.25 I(X1;Y)=0.000000 I(X2;Y)=0.000000 I(pair;Y)=0.188722
flip=0.50 I(X1;Y)=0.000000 I(X2;Y)=0.000000 I(pair;Y)=0.000000
checks: 51 probability tables agree with 1 - binary_entropy(flip)
