theorem
  X1,X2 are_isomorphic & X1 is proper-for-identity implies
  X2 is proper-for-identity
  proof
    given F,G such that
A1: F,G form_isomorphism_between X1,X2;
    assume
A2: X1 is proper-for-identity;
    let s1,s2 be stack of X2;
A3: dom G = the carrier' of X1 & rng G = the carrier' of X2 by A1; then
    consider q1 being object such that
A4: q1 in dom G & s1 = G.q1 by FUNCT_1:def 3;
    consider q2 being object such that
A5: q2 in dom G & s2 = G.q2 by A3,FUNCT_1:def 3;
    reconsider q1,q2 as stack of X1 by A1,A4,A5;
A6: dom F = the carrier of X1 & rng F = the carrier of X2 &
    F is one-to-one by A1;
A7: rng |.q1.| c= the carrier of X1 & rng |.q2.| c= the carrier of X1;
    assume |.s1.| = |.s2.|; then
A8: F*|.q1.| = |.s2.| by A1,A4,Th45 .= F*|.q2.| by A1,A5,Th45;
    dom (F*|.q1.|) = dom |.q1.| & dom (F*|.q2.|) = dom |.q2.|
    by A6,A7,RELAT_1:27; then
    |.q1.| = |.q2.| by A6,A7,A8,FUNCT_1:27; then
    q1 == q2;
    hence thesis by A2,A4,A5;
  end;
