Sample program for lab 4

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1:31f0f53b08bd
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3:d5c746840139
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+++ b/mbed-os.lib	Wed Feb 07 16:56:55 2018 +0000
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+https://github.com/armmbed/mbed-os/#caeaa49d68c67ee00275cece10cd88e0ed0f6ed3