In mathematical logic we have iff: if and only if. As an example of this for my classes I realised that in sworn oaths there are three statements for telling:
1: the truth,
2: the whole truth, and
3: nothing but the truth.
The first two of these I rewrite as follows:
Statement 1: If I say something, then that something is true.
Statement 2: If something is true, then I will say that something.
Together these two statements are enough, since they imply what you say if and only if it is true. Hence the third statement is superfluous.
So my questions is: why is this third phrase included in the oath?