Spec suggestion: spell out serialization parsing context

A GRDDL transform transforms a document to a graph.  Transform authors
typically transform a document to a serialization of graph.

Conceptually what is typically happening is in two stages:

1. Transform the document to a serialization of the graph
2. Parse that serialization 

Parsing occurs in a context, e.g. same document references and base URI
(I can't think of any others off the top of my head, but their might be
some).

Should the spec spell out that context so that transform authors know
what they can rely on?

Brian