reserve Omega, F for non empty set,
  f for SetSequence of Omega,
  X,A,B for Subset of Omega,
  D for non empty Subset-Family of Omega,
  n,m for Element of NAT,
  h,x,y,z,u,v,Y,I for set;

theorem Th2:
  for a,b being set holds Union (a,b) followed_by {} = a \/ b
proof
  let a,b be set;
  rng (a,b) followed_by {} = {a,b,{}} by FUNCT_7:127;
  hence Union ((a,b) followed_by {}) = union{a,b} by MEASURE1:1
    .= a \/ b by ZFMISC_1:75;
end;
