'append the given correlation expression to this select()
construct.
This is an **in-place** mutation method; the
:meth:`~.Select.correlate` method is preferred, as it provides standard
:term:`method chaining`.'

def append_correlation(self, fromclause):

    self._auto_correlate = False
    self._correlate = set(self._correlate).union((_interpret_as_from(f) for f in fromclause))
