Declare and set a project global constant, whose value is a path. The
        path is adjusted to be relative to the invocation directory. The given
        value path is taken to be either absolute, or relative to this project
        root.

def path_constant(self, name, value):
        """Declare and set a project global constant, whose value is a path. The
        path is adjusted to be relative to the invocation directory. The given
        value path is taken to be either absolute, or relative to this project
        root."""
        assert is_iterable_typed(name, basestring)
        assert is_iterable_typed(value, basestring)
        if len(value) > 1:
            self.registry.manager.errors()("path constant should have one element")
        self.registry.current().add_constant(name[0], value, path=1)