reserve i,j for Nat;
reserve x,y for set;
reserve A for non empty set;
reserve c for Element of StandardStackSystem A;
reserve m for stack of StandardStackSystem A;
reserve X for non empty non void StackSystem;
reserve s,s1,s2 for stack of X;
reserve e,e1,e2 for Element of X;
reserve X for StackAlgebra;
reserve s,s1,s2,s3 for stack of X;
reserve e,e1,e2,e3 for Element of X;
reserve X1,X2,X3 for StackAlgebra;
reserve F,F1,F2,G,G1,G2 for Function;

theorem
  X1,X2 are_isomorphic & X2,X3 are_isomorphic implies X1,X3 are_isomorphic
  proof
    given F1,G1 such that
A1: F1,G1 form_isomorphism_between X1,X2;
    given F2,G2 such that
A2: F2,G2 form_isomorphism_between X2,X3;
    take F2*F1, G2*G1;
    thus thesis by A1,A2,Th44;
  end;
