Identify library module.
A ModuleDecl identifies a module as a library module, since
only library modules contain a module declaration. It contains the keyword
module which is followed by a namespace declaration with the
NCName specifying the namespace prefix and the string literal specifying the
namespace URI that defines the target namespace of the library module. This
namespace is used for all functions and variables exported by this module.
This module declaration binds the URI http://example.org/math-functions to all functions and variable declarations that follow this declaration:
module namespace math = "http://example.org/math-functions"
The following construct(s) refer to this construct:
