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 Th51:
  5 = { 0,1,2,3,4 }
proof
  thus 5 = succ 4
    .= { 0,1,2,3,4 } by Th50,ENUMSET1:10;
end;
