# Day 11: McCarthy 91

Our today’s guest is very special. No doubt it is one of the most useless functions ever, yet, the most profound for causing nightmares of computer science students.

What does is do? When you plug a natural number `N` in, McCarthy91 flushes `N-10` out, if `N` is larger than `100`. Otherwise it always returns `91`.

Why should anybody have nightmares? Well, this function is being used as a study case for formal verification. In other words, you don’t have to implement it, you have to prove it is correct and finite, instead.

#### algorithm

https://github.com/coells/100days

`def mccarthy91(n):    k = 1    while k:        if n > 100:            n -= 10            k -= 1        else:            n += 11            k += 1    return n`
`def mccarthy91_rec(n):    if n > 100:        return n - 10    else:        return mccarthy91_rec(mccarthy91_rec(n + 11))`

#### test

`> mccarthy91(80)`
`91`
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