The IIOMetadataFormat class supports a number of "child policies," which
describe the legal sequences of child nodes for a particular element node.
These patterns are a restricted subset of the possible patterns describable
in an XML DTD.

  In order to support the possible formats of JPEG metadata, and other future
formats, a new child policy is required that corresponds to a DTD regular
expression of the form (a | b | c | d)*. That is, any sequence of
child nodes of types a, b, c, or d.

  To implement this, a new constant IIOMetadataFormat.CHILD_POLICY_SEQUENCE
should be defined. An additional constant, CHILD_POLICY_MAX should be defined
which takes on the largest value from among the other constants, in order
to insulate range-checking code from future expansion of the set of child
policies.

  The methods in IIOMetadataFormatImpl that perform range checking on
childPolicy arguments should be modified to test against CHILD_POLICY_MAX.