The number of extra rows needed for each of our inputs to compute this
        term.

def dependencies(self):
        """
        The number of extra rows needed for each of our inputs to compute this
        term.
        """
        extra_input_rows = max(0, self.window_length - 1)
        out = {}
        for term in self.inputs:
            out[term] = extra_input_rows
        out[self.mask] = 0
        return out