Notation perm_eql s1 s2 := (perm_eq s1 =1 perm_eq s2).Notation perm_eqr s1 s2 := (perm_eq^~ s1 =1 perm_eq^~ s2).

Lemma perm_eq_small s1 s2 : size s2 <= 1 -> perm_eq s1 s2 -> s1 = s2.