Command-base programming

Learn about `Command-base programming`, a design pattern supported by WPILib that organizes a robot program.
- Understand the basics of two fundamental concepts, `subsystems` and `commands`
- Understand the difference between subsystems and commands
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Further studies: http://wpilib.screenstepslive.com/s/4485/m/13809/l/241892-what-is-command-based-programming

Start with the linked WPILib material on command-based programming. Review the basics of subsystems and commands and explain how they differ; the work is done when the learning material or notes clearly cover both concepts and their relationship.

Written by the indexing model from the issue text.