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+## What's mrbgems?
+
+(To be written...)
+
+## How to add mrbgems?
+
+(To be written...)
+
+## My mruby don't have.
+
+### Kernel#sleep
+
+See https://github.com/matsumoto-r/mruby-sleep
+
+### Random class
+
+https://github.com/matsumoto-r/mruby-random
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