Definition cfclass (A : {set gT}) (phi : 'CF(A)) (B : {set gT}) :=
[seq (phi ^ repr Tx)%CF | Tx in rcosets 'I_B[phi] B].
Local Notation "phi ^: G" := (cfclass phi G) : cfun_scope.

(* This is Isaacs, Corollary (6.7). *)
Corollary constt0_Res_cfker i :
H <| G -> 0 \in irr_constt ('Res[H] 'chi[G]_i) -> H \subset cfker 'chi[G]_i.