This introduces a new node type, ReceiverParameter. This is because a receiver parameter is not a formal parameter, even though it looks like one: it doesn't declare a variable, and doesn't affect the arity of the method or constructor. It's so rarely used that giving it its own node avoids matching it by mistake and simplifies the API and grammar of the ubiquitous FormalParameter and VariableDeclaratorId.

This also simplifies the few places where we pay attention to the receiver parameter --- to ignore it.

So for example:

(@A Foo this, Foo other)

FormalParameters
  + FormalParameter[@ReceiverParameter=true()]
    + ClassOrInterfaceType
       + Annotation "A"
    + VariableDeclaratorId[@Image="this", @ReceiverParameter=true()]
  + FormalParameter
    + ClassOrInterfaceType
    + VariableDeclaratorId "other"

FormalParameters
  + ReceiverParameter
    + ClassOrInterfaceType
       + Annotation "A"
  + FormalParameter
    + ClassOrInterfaceType
    + VariableDeclaratorId "other"