Use cases of types

[civp]

We can provide two use cases

• Implement an even safer language checker based on semantic analysis, forbidding the usage of blacklisted methods such as List#head and Option#get
• Implement an even purer functional language checker based on semantic analysis, forbidding the usage of blacklisted methods such as println and IterableOnceOps#foreach

For each method, we need to know the type of the object. We also need subtyping information. For example, if IterableOnceOps#foreach is blacklisted, then all the methods of the form A#foreach, where A is a subclass of IterableOnceOps, should be disallowed.