Problem 1: Mobiles (400pts)

Acknowledgements:

This mobile example is based on a classic problem from Structure and Interpretation of Computer Programs, Section 2.2.2.

We are making a planetarium mobile. A mobile is a type of hanging sculpture. A binary mobile consists of two arms. Each arm is a rod of a certain length, from which hangs either a planet or another mobile.

We will represent a binary mobile using the data abstractions below:

• A mobile has a left arm and a right arm.
• An arm has a positive length and something hanging at the end, either a mobile or planet.
• A planet has a positive size.