reserve A,B for Ordinal,
  K,M,N for Cardinal,
  x,x1,x2,y,y1,y2,z,u for object,X,Y,Z,X1,X2, Y1,Y2 for set,
  f,g for Function;
reserve m,n for Nat;
reserve x1,x2,x3,x4,x5,x6,x7,x8 for object;

theorem Th49:
  card {x1,x2} <= 2
proof
A1: {x1,x2} = {x1} \/ {x2} & 1+1 = 2 by ENUMSET1:1;
  card {x1} = 1 & card {x2} = 1 by CARD_1:30;
  hence thesis by A1,Th42;
end;
