reserve x,y,z,w for ExtReal,
  r for Real;
reserve f,g for ExtReal;
reserve t for ExtReal;

theorem
  x/2 + x/2 = x
proof
  x/2 + x/2 = (x+x)/2 by Th95
    .= 2*x/2 by Th94
    .= x*(2/2) by Th66
    .= x by Th81;
  hence thesis;
end;
