*Document has structural element type if and only if
that document is some standard
where structural element type = standard
or that document is some handbook
where structural element type = handbook
or that document is some technical memorandum
where structural element type = technical memorandum.
PermanentFact = False
HiddenFact = False
InternalUniquenessConstraint71
InternalUniquenessConstraint
Each document has at most one structural element type.