defining lambda expressions I am studying for my exam and I wanted to do some extra excercises, but I have some problems with solving :) Can anyone please help or give me some advice where to start? Thank you!

We can represent every list of lambda expressions $x_0, x_1, ..., x_k$ with lambda expression $[x_0, x_1, ..., x_k]$ , defined as:
$[x_0, x_1, ... , x_k] = \lambda c n. c \, x_0 \, (c \, x_1 \, (... (c \, x_k \, n) ...))$ 

Define lambda expression s, satisfying the equation  $s([\underline{n_0}, ..., \underline{n_k}]) = \underline{n_0 + ... + n_k}$ , where  $\underline{n}$ is a Church numeral, representing natural number $n$.

Define lambda expression $r$ , satisfying the equation  $r([x_0, ..., x_k]) = [x_k,  ..., x_0]$ .

Define lambda expression $h$ and $t$ , satisfying the equation  $h([x_0, ..., x_k]) = x_0 in t([x_0, x_1, ..., x_k]) = [x_1, ..., x_k]$ .
Hint: This way lists are represented as folds. So if you have a lambda term representing a list, you just supply a folding function and a value to start with and the result will be the folded value.