This attribute is a string which is to be used as the namespace
prefix for all XML elements created as a result of writing an AST
Object out through an XmlChan. The URI associated with the namespace
prefix is given by the symbolic constant AST__XMLNS defined in
A definition of the namespace prefix is included in each top-level
element produced by the XmlChan.
The default value is a blank string which causes no prefix to be
used. In this case each top-level element will set the default
namespace to be the value of AST__XMLNS.