Frame (context) data passed to each frame parsing virtual methods. In
addition to providing the data to be checked for a valid frame or an already
identified frame, it conveys additional metadata or control information
from and to the subclass w.r.t. the particular frame in question (rather
than global parameters). Some of these may apply to each parsing stage, others
only to some a particular one. These parameters are effectively zeroed at start
of each frame's processing, i.e. parsing virtual method invocation sequence.