reserve A,B,C for Ordinal,
  X,X1,Y,Y1,Z for set,a,b,b1,b2,x,y,z for object,
  R for Relation,
  f,g,h for Function,
  k,m,n for Nat;
reserve M,N for Cardinal;
reserve S for Sequence;

theorem Th52:
  6 = { 0,1,2,3,4,5 }
proof
  thus 6 = succ 5
    .= { 0,1,2,3,4,5 } by Th51,ENUMSET1:15;
end;
