Section Huffman.
Variable A : Type.
Variable empty : A.
Variable eqA_dec : forall a b : A, {a = b} + {a <> b}.
Variable m : list A.
Hypothesis frequency_more_than_one : 1 < length (frequency_list eqA_dec m).

(* The message is not null *)
Theorem not_null_m : m <> nil.