In env.py:

#Decorator for overriding builtins
def return_subclass_from(specials):
  specialnames = ['__%s__' % s for s in specials.split()]
  def wrapit(cls, method):
	return lambda *a: cls(method(*a))
  def dowrap(cls):
	for n in specialnames:
	  method = getattr(cls, n)
	  setattr(cls, n, wrapit(cls, method))
	return cls
  return dowrap

#The real number type
@return_subclass_from('abs add floordiv mod mul neg pow radd rdivmod rfloordiv rmod rmul rpow rsub rtruediv sub truediv trunc')
class Real(Fraction):