How to use the micromagneticmodel.Exchange function in micromagneticmodel

import numbers
import oommfc.util as ou
import discretisedfield as df
import micromagneticmodel as mm


class Exchange(mm.Exchange):
    """Exchange energy term.

    This energy term models ferromagnetic exchange energy term,
    defined by the exchange energy constant `A`.

    .. math::

           w_{ex} = A(\\nabla\\mathbf{m})^{2}

    `A` is a scalar value with J/m units.

    `A` can be either contant in space or spatially varying. If it is
    constant, a single value is passed, e.g. `A = 1e-12`. On the other
    hand, if it varies in space, there are two ways how that can be
    defined. The first one is using a dictionary, where the keys must
    be the same as the names of regions used when the mesh was defined