### The task

Three neighbouring elements of an integer list form a sum triplet or a
difference triplet, respectively, if the sum or the difference,
respectively, of the first and the third elements of the triplet equals to
the middle element of the triplet.
Write an SML function called `sumdiff`

that returns true if and only if the
list passed in its argument contains at least one sum triplet or difference
triplet.

Write declarative head comments to any auxiliary function you wish
to define.
### Examples:

sumdiff [] = false;
sumdiff [1,1] = false;
sumdiff [1,2,4] = false;
sumdiff [1,2,~1] = true;
sumdiff [1,2,3,~1] = true;
sumdiff [1,2,3,~1,2] = true;
sumdiff [1,2,4,~3,4] = false;