reserve m,n for Nat;
reserve i,j for Integer;
reserve S for non empty addMagma;
reserve r,r1,r2,s,s1,s2,t,t1,t2 for Element of S;
reserve G for addGroup-like non empty addMagma;
reserve e,h for Element of G;
reserve G for addGroup;
reserve f,g,h for Element of G;
reserve u for UnOp of G;
reserve A for Abelian addGroup;
reserve a,b for Element of A;
reserve x for object;
reserve y,y1,y2,Y,Z for set;
reserve k for Nat;
reserve G for addGroup;
reserve a,g,h for Element of G;
reserve A for Subset of G;
reserve G for non empty addMagma,
  A,B,C for Subset of G;
reserve a,b,g,g1,g2,h,h1,h2 for Element of G;
reserve G for addGroup-like non empty addMagma;
reserve h,g,g1,g2 for Element of G;
reserve A for Subset of G;
reserve H for Subgroup of G;
reserve h,h1,h2 for Element of H;
reserve G,G1,G2,G3 for addGroup;
reserve a,a1,a2,b,b1,b2,g,g1,g2 for Element of G;
reserve A,B for Subset of G;
reserve H,H1,H2,H3 for Subgroup of G;
reserve h,h1,h2 for Element of H;

theorem Th55:
  G1 is Subgroup of G2 & G2 is Subgroup of G1 implies the
  addMagma of G1 = the addMagma of G2
proof
  assume that
A1: G1 is Subgroup of G2 and
A2: G2 is Subgroup of G1;
  set g = the addF of G2;
  set f = the addF of G1;
  set B = the carrier of G2;
  set A = the carrier of G1;
A3: A c= B & B c= A by A1,A2,DefA5;
A4: A = B by A1,A2,DefA5;
  f = g||A by A1,DefA5
    .= (f||B)||A by A2,DefA5
    .= f||B by A4,RELAT_1:72
    .= g by A2,DefA5;
  hence thesis by A3,XBOOLE_0:def 10;
end;
