Deal with plane convex sets. A plane convex set, subsequently abbreviated to "convex set," is a nonempty set X in the plane having the property that if x and y are any two points in X, the straight-line segment from x to y is also in X. The following figures illustrate.
Suppose that X1, X2, X3, X4 are convex sets, each three of which have a common point. Prove that all four sets have a common point.
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