Return the correct slice for a weights/mask matrix at this
        ConnectionField's location on the sheet (i.e. for getting the
        correct submatrix of the weights or mask in case the unit is
        near the edge of the sheet).

def positionlesscrop(self,x,y,sheet_coord_system):
        """
        Return the correct slice for a weights/mask matrix at this
        ConnectionField's location on the sheet (i.e. for getting the
        correct submatrix of the weights or mask in case the unit is
        near the edge of the sheet).
        """
        slice_inds = self.findinputslice(
            sheet_coord_system.sheet2matrixidx(x,y),
            self.shape_on_sheet(), sheet_coord_system.shape)

        self.set(slice_inds)