piecewise_fold of nested piecewise expressions; bug?

[clayto...@gmail.com]

Hello,

From the sympy master, I have tried to simplify a piecewise expression which contains piecewise arguments:

from sympy import Piecewise, Eq, piecewise_fold
from sympy.abc import x, y

p = Piecewise((Piecewise((5, Eq(y, 0)), (2, True)), Eq(x, 0)), (Piecewise((1, Eq(y, 0)), (6, True)), True))
print(piecewise_fold(p))

I would expect that the result has only one "Piecewise" in it, something like:

Piecewise((5, And(Eq(y, 0), Eq(x,0))), (2, And(True, Eq(x, 0))), (1, And(Eq(y, 0), True)), (6, And(True, True))) # Expected result.

or, if simplified,

Piecewise((5, And(Eq(y, 0), Eq(x,0))), (2, Eq(x, 0)), (1, Eq(y, 0)), (6, True)) # Expected result.

But instead, the different piecewise subexpressions are not folded together.  Is this a bug, an unimplemented feature, or have I misunderstood the purpose of piecewise_fold?

Thanks,

Clayton

PS.  This testcase arose from a multiple integration which was not fully simplified -- so although it looks artificial, something equivalent did arise in actual usage.

[Aaron Meurer]

It's probably an unimplemented feature. There are a lot of things that
piecewise_fold can't handle which it could.

Aaron Meurer

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[clayto...@gmail.com]

OK Aaron -- thanks for your reply.  I'll see if I can generate a patch...