Proof with auto.
unfold compose.
unfold RsumOver.
induction l.
simpl.
apply Rmult_0_r.
simpl.
rewrite <- IHl.
apply Rmult_plus_distr_l.
Qed.

Lemma RsumOver_minus w f b d: (b + w <= d)%nat ->
RsumOver (nats b w) (f ∘ minus d) = RsumOver (nats (S (d - (w + b))) w) f.