For any three sets $P$,$Q$ and $R$, $s$ is an element of $(P \bigtriangleup Q)\bigtriangleup R$ if $s$ is in
- Exactly one of $P$,$Q$ and $R$
- at least one of $P,Q,R$ but not in all three of them at the same time
- exactly two of $P,Q$ and $R$
- exactly one of $P,Q$ and $R$ or in all the tree of them.