FieldReferenceLinker

It is assumed that yield invocation for inspection will never be nested, since a query is completely built (and it's yield inspection is done) before it can be nested, this is unlikely to change, but documenting this assumption was deemed useful, because this method would stop working (without complaining) if (the assumption) was broken.

_lastAccessedFieldReference is unique per thread, AST construction can be nested and can interfere with one another, this method is used for preserving the previous _lastAccessedFieldReference when a nested AST construction takes place and during the construction of 'sample' POSOs, because they are proxied, and can call their intercepted fields during construction, calling the constructor for 'sample' POSO construction without wrapping with this methor would have the effect of 'polluting' the _lastAccessedFieldReference (issue 68).