This is a node which defined on different subsets differently. When evaluating, it will turn
into a particular case once all cases' predicates before are false, and this case's predicate is true.

That is, the order counts. An example:
Piecewise(a provided false, b provided true, c provided true)
Will be evaluated into b,

Piecewise(a provided false, b provided c, c provided false)
Will remain as it is (although unreachable cases will be removed)