Parses a :ref:`constraints_expression <constraints-expressions>`
        and returns a :class:`Constraints` object.

def parse(cls, constraints_expression):
        """Parses a :ref:`constraints_expression <constraints-expressions>`
        and returns a :class:`Constraints` object.
        """
        constraint_exprs = re.split(r'\s*,\s*', constraints_expression)
        return Constraints(merge(Constraint.parse(constraint_expr)
                                 for constraint_expr in constraint_exprs))