Every idref in an attribute must point to the @xml:id
value of the appropriate corresponding element.
Affects: @who
@ed-who
@roles
@src
@lexicon
@morphology
@reuse-type
@bitext-relation
@feature
@include
Used by template ŧ resolve-attr-include
ŧ core-errors
ŧ core-attribute-errors
Used by function tan:resolve-doc
() tan:idrefs-loop
()