# More on permutations

Consider the notion of permutation as defined in
that exercise

Build a couterexample, with two lists *l* and
*l'* of the same length, such that the proposition
`perm l l'` holds. Prove this proposition.

### Hint

Consider lists on `nat` or `Z`, or any type suitable
for defining a function which counts the number of occurrences of
any item in a list. Then prove and apply a lemma stating that if
*l'* is a permutation of *l*, then this function always returns the same value when applied to *l* or *l'*.
## Solution

Follow this link

Going home

Pierre Castéran