'Helper method for set_coordinate_type. It calculates inverse
transformation equations for given transformations equations.
Parameters
equations : tuple
Tuple of transformation equations'

def _calculate_inv_transformation_equations(self, equations):

    x = Dummy('x')
    y = Dummy('y')
    z = Dummy('z')
    eq = self._transformation_equations()
    try:
        solved = solve([(eq[0] - x), (eq[1] - y), (eq[2] - z)], [x, y, z], dict=True)[0]
        return (solved[x], solved[y], solved[z])
    except:
        raise ValueError('Wrong    set    of    parameters.')
