Remove singularities

[LeonardJohard]

In some rare cases adding another variable and equation pair, even when they are an isolated and consistent system, makes the remove singularity functionality add one equation and unbalance the equation system.

I will try to nail down this problem into an example I can share.

[ajbergqvist]

It sounds similar to an issue I've had. The problem can be reproduced with this simplified model:

using Modia
@model testModel begin
k=1e-4
b1 =Float()
b2 =Float()
b3 =Float()
bav=Float()
V=Float()
@equations begin
k * der(b1) = b2 - 2 * b1 + b2
k * der(b2) = b3 - 2 * b2 + b1
bav = (b1+b2+b3)/3.0
bav = 1.0 * sin(2 * pi * 1000.0 * time)
# Some alternatives for an equation for V, uncomment one:
V=1.0 * der(bav) # Fails
#V=1.0 * der(b1)+1.0 * der(b2)+1.0 * der(b3) # Fails
#V=1.0 * der(b1)+1.0 * der(b2)+0.0 * der(b3) # Fails
#V=1.0 * der(b1)+1.0 * der(b2) # Runs
#V = der(bav) # Fails with different message
end
end;
r=simulate(testModel,5e-3);

The equivalent Modelica code runs without issues in OpenModelica (with all alternatives for "V" equation).