There was an inconsistency between two tests that created a problem.  One test in test_integrals expected

    Integral(exp(-x**2 + 5), x).subs(x,x) == Integral(exp(-x**2+5), (x,x) )

and another doctest in integral.py expected

	Integral(x + y, y, (y, 1, x)).diff(x) == Integral(x + y, (y, x)) + Integral(1, y, (y, 1, x))

(note the single y term in the last integral.  `diff()` makes implicit use of `subs()` ).  Since there seemed to be no change that preserved both possibilities, I picked the second as more efficient.