This allows basic static validation of a traversal against a schema, to
verify that it represents a valid path through the structural parts of
the schema.
The main purpose of this is to produce better error messages (using our
knowledge of the schema) than we'd be able to achieve by just relying
on HCL expression evaluation errors. This is particularly important for
nested blocks because it may not be obvious whether one is represented
internally by a set or a list, and incorrect usage would otherwise produce
a confusing HCL-oriented error message.
Martin Atkins