Abstract interface to perform some operations on AST nodes and possibly get some result.

For every subtype of Node, there is one transform method, each returning Result. These are all formal, as there are two sensible default behaviors:

• widening, as done in WideningTransformer: every transform eventually ends up in transformNode().
  • for example: transformTypeAliasDefinition() → transformTypeDeclaration() → transformDeclaration() → transformNode()
• narrowing, as done in NarrowingTransformer: every transform eventually ends up in the transform method for the concrete class.
  • for example: transformIdentifier() switches on Identifier’s case types and delegates to the transform methods for the subclasses.

Be careful when mixing the two behaviors, lest you end up in an infinite recursion!