Thanks for the error report. This can be filed as an issue on our
issue tracker: http://code.google.com/p/sympy/issues

A workaround is to use fraction() to break up the original fraction
since this is not recursive like as_numer_denom. fraction will give
the literal numerator and denominator of an expression; numer and
denom are methods that can also target a given part of the fraction:

>>> fraction(f)
(x + y, a/b + y)
>>> numer(f).subs(a/b,z)/denom(f).subs(a/b,z)
(x + y)/(y + z)

/Chris

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