'Decrease the level of self.
This makes the working differential extension smaller.  self.level is
given relative to the end of the list (-1, -2, etc.), so we don\'t need
do worry about it when building the extension.'

def decrement_level(self):

    if (self.level <= (- len(self.T))):
        raise ValueError('The    level    of    the    differential    extension    cannot    be    decremented    any    further.')
    self.level -= 1
    self.t = self.T[self.level]
    self.d = self.D[self.level]
    self.case = self.cases[self.level]
    return None
