Fix a very subtle bug which occurs in piecewise_if.
Fix a very subtle bug which occurs in piecewise_if.

For input like `piecewise_if(cond, th, piecewise(cond1,th1))' it could produce
`piecewise(cond,th, cond1,th1)' which is not equivalent to the original
expression, since `piecewise(c,t)' has an implicit `otherwise -> 0' case.

zachsully/hakaru,zaxtax/hakaru,zaxtax/hakaru,zachsully/hakaru