The root node of a DDI 2.5 Codebook file. This file must contain stdyDscr.
More information on this element, especially the allowed attributes, can
be found in the references.
# All ddi_codeBook() functions must contain ddi_stdyDscr(), # which also has ddi_citation() as a required child element.
ddi_codeBook(ddi_stdyDscr(ddi_citation()))