Inverts the ScienceSegments in the class (i.e. set NOT).  Returns the
    number of ScienceSegments after inversion.

def invert(self):
    """
    Inverts the ScienceSegments in the class (i.e. set NOT).  Returns the
    number of ScienceSegments after inversion.
    """

    # check for an empty list
    if len(self) == 0:
      # return a segment representing all time
      self.__sci_segs = ScienceSegment(tuple([0,0,1999999999,1999999999]))

    # go through the list checking for validity as we go
    outlist = []
    ostart = 0
    for seg in self:
      start = seg.start()
      stop = seg.end()
      if start < 0 or stop < start or start < ostart:
        raise SegmentError, "Invalid list"
      if start > 0:
        x = ScienceSegment(tuple([0,ostart,start,start-ostart]))
        outlist.append(x)
      ostart = stop

    if ostart < 1999999999:
      x = ScienceSegment(tuple([0,ostart,1999999999,1999999999-ostart]))
      outlist.append(x)

    self.__sci_segs = outlist
    return len(self)