Semantics
A package-begin statement
that names a package p
sets the current package of the
schema context to p. There can be
at most one current package in
the schema context at any given time.
If the package-begin statement
is evaluated when the schema context already has a
current package, the statement is
rejected with an error.
If the package-begin statement
is evaluated with package name r,
where r is already defined within
the schema context, the statement is rejected with an error.