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X1 X2 (X1,X2) (X1,X2,X3,X4)
x1:x2:x3=:1:2 x1:x2:x3=2:1: x1:x2:x3=1:2: x1:x2:x3=:2:1
X1 X2 (X1,X2) (X1,X2,X3,X4)
X1 X2 (X1,X2) (X1,X2,X3,X4)
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X1 X2 (Xl,X2) (Xl,X2,X3,X4)