This is an internal function that is called from R's C code to
  determine the enclosing namespace of a
  .C/.Call/.Fortran/.External call which has
  no PACKAGE argument. If the call has been made from a function
  within a namespace, then we can find the DLL associated with that
  namespace.  The purpose of this is to avoid having to use the
  PACKAGE argument in these native calls and so better support
  versions of packages.
This is an internal function that may be migrated to internal C
  code in the future and so should not be used by R programmers.