Variable index model gives incorrect results

[Francesco Casella]

The following model

model foo
  Real x1, x2;
equation
  der(x1) = 2;
  if x1 < 1 then 
    der(x2) = 1;
  else
    x2 = 1;
  end if;
end foo;

simulated for one second gives as results x1 = 2*time (which is correct) and x2 = time, which is wrong for time > 0.5.

I understand an error should be generated at time = 0.5, or maybe right at compile time, unless we can handle variable structure systems, which we obviously can't (or can we?).