def METHOD_NAME(self, layername, grassmapname, force=False):
    """
    Put an existing map from GRASS to the layer collection
    @param string name of the layer
    @param string name of an existing GRASS map layer
    @param boolean optional, whether to overwrite values if key exists
    """
    # fill the new grass array with the contents from the map (must exist)
    if grassmapname in grass.list_strings("rast"):
        layer = garray.array(grassmapname)
        self.grassmapnames[layername] = grassmapname
        self.setlayer(layername, layer, force)
    else:
        raise error.DataError(
            Grassland.ME, "Grass Map was missing: " + grassmapname
        )