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 Th50:
  4 = { 0,1,2,3 }
proof
  thus 4 = succ 3
    .= { 0,1,2,3 } by Th49,ENUMSET1:6;
end;
