Declare options that are valid for xml_in and xml_out.

{
'in'  => %w(
  keyattr keeproot forcecontent contentkey noattr
  searchpath forcearray suppressempty anonymoustag
  cache grouptags normalisespace normalizespace
  variables varattr keytosymbol
),
'out' => %w(
  keyattr keeproot contentkey noattr rootname
  xmldeclaration outputfile noescape suppressempty
  anonymoustag indent grouptags noindent
)