no support for discrete equation systems

[Lennart Ochel]

Some discrete variables may be involved in equation systems. This is currently not supported.

model foo
  Real a;
initial equation 
  pre(a)^2 + a^2 = 9;
equation 
  when {sample(0.1, 0.1)} then
    a = time;
  end when;
end foo;

Error log:

Translation | Interner Fehler Transformation Module sort components failed!
Translation | Interner Fehler sorting equations(strongComponents failed)
Translation | Interner Fehler ./Compiler/BackEnd/BackendDAETransform.mo: function analyseStrongComponentBlock failed
Translation | Interner Fehler Sorry - Support for Discrete Equation Systems is not yet implemented
              $PRE.a
              a
              $PRE.a ^ 2.0 + a ^ 2.0 = 9.0
              a = $PRE.a