- string literals with backslash escapes (maybe in lexer?)
- adjustment for methods, classes, and interfaces:
    methods:
      optional modifiers, void type
    classes, interfaces:
      optional modifiers, optional extends/implements...
    generic interfaces, methods:
      order of concrete children (DONE)
- compilation unit: push down comments into first type
    (careful: optional package decl and imports)
- pretty printing of all node types