docs: def deltas(self):
  		"""
  		Dictionary of relative offsets.  The keys in the result are
  		pairs of keys from the offset vector, (a, b), and the
  		values are the relative offsets, (offset[b] - offset[a]).
  		Raises ValueError if the offsetvector is empty (WARNING:
  		this behaviour might change in the future).
  
  		Example:
  
  		>>> x = offsetvector({"H1": 0, "L1": 10, "V1": 20})
  		>>> x.deltas
  		{('H1', 'L1'): 10, ('H1', 'V1'): 20, ('H1', 'H1'): 0}
  		>>> y = offsetvector({'H1': 100, 'L1': 110, 'V1': 120})
  		>>> y.deltas == x.deltas
  		True
  
  		Note that the result always includes a "dummy" entry,
  		giving the relative offset of self.refkey with respect to
  		itself, which is always 0.
  
  		See also .fromdeltas().
  
  		BUGS:  I think the keys in each tuple should be reversed.
  		I can't remember why I put them in the way they are.
  		Expect them to change in the future.
  		"""
  		# FIXME:  instead of raising ValueError when the
  		# offsetvector is empty this should return an empty
  		# dictionary.  the inverse, .fromdeltas() accepts
  		# empty dictionaries
  		# NOTE:  the arithmetic used to construct the offsets
  		# *must* match the arithmetic used by
  		# time_slide_component_vectors() so that the results of the
  		# two functions can be compared to each other without worry
  		# of floating-point round off confusing things.
  		refkey = self.refkey
  		refoffset = self[refkey]
  		return dict(((refkey, key), self[key] - refoffset) for key in self)