This program illustrates the use of the STL stable_partition() algorithm to
partition the Point values in a vector of Points into two groups: those whose
x-coordinate is divisible by 3, and those for whom this is not true.

An interesting exercise is to change the call to stable_partition() to a call
to partition() and observe the difference in output.
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Here are the contents of the vector:
(1,2) (3,4) (4,5) (6,7)
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And here are the contents of the partitioned vector:
(3,4) (6,7) (1,2) (4,5)
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The iterator p returned by the algorithm points to the value (1,2).
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