Extract a section element from an Rd object or Rd fragment.
Rdo_section(rdo, sec)An Rd fragment for use as a section element of an Rd object
an Rd object or fragment
the required section, a string
Georgi N. Boshnakov
If the class or the "Rd_tag" attribute of rdo is "Rd" the
required section is extracted. Otherwise, if this attribute is equal to
sec, then rdo is returned.
In all other cases it is assumed that rdo is the contents of
the required section, its "Rd_tag" attribute is set to sec and
returned without further modification.