Inverse solving of algorithms

[Lennart Ochel]

There is one model in the test suite that cannot get initialized with the symbolic initialization, since there is no support for inverse solving of algorithms:

testsuite/simulation/libraries/msl31/Modelica.Mechanics.MultiBody.Examples.Loops.PlanarLoops_analytic.mos

The following model can be used as simplified test case:

model A
  Real x, y, z;
  Real der_x, der_y, der_z;
equation
  der_x = der(x) + 1;
  der_y = der(y) + 2;
  der_z = der(z) + 3;
initial equation
  der(x) = 0;
  der(y) = 0.5;
  der(z) = 0;
algorithm
  der_x := x;
  der_y := der_x + y;
  der_z := der_y + z;
end A;

The output is the following:

Error: Internal error Inverse Algorithm needs to be solved for {y,z,x} in 
algorithm
  der_x := x;
  der_y := der_x + y;
  der_z := der_y + z;
This has not been implemented yet.
Error: Internal error function createSingleAlgorithmCode failed!
Error: Internal error function createOdeSystem failed for component Algorithm 4 {7, 5, 9}
Error: Internal error function createEquationsForSystem1 failed for component Algorithm 4 {7, 5, 9}
Warning: No system for the symbolic initialization was generated. A method using numerical algorithms will be used instead.