Any top-level, optional element is allowed to appear more
than once - Any extensibility element is allowed in any place.
Such extensibility elements must be in the substitution group of
globalExt or postTypes
Either the element attribute OR the type attribute may appear but not both.
Alternatively a qualified extensibility attribute which refers to constructs
in some type system other than XML Schema may be used.
This abstract type is intended to serve as the base type for
extensibility elements. It includes the wsdl:required attribute
which it is anticipated will be used by most extension elements