Input to linearizer is tree
Tree specifies DTD/SGML declaration with respect to which
validation is to be performed
Validation is performed with respect to an information set.
Validity is defined on linear form: conceptually linearizer creates
linear form which it parses and validates, creates a new tree using
same information set, and then checks that the resulting tree is the
same as the original.
Linearizing not intelligent: only adds nodes and properties not in
information set. (Probably too restrictive: don't want to require
applications to create all implied attributes.)
Output DTD may be augmented by generating declarations for entities or
notations that occur in the tree.