the `NDSolvevalue`

MMA can solve finite element problems well according to displacement limit conditions

```
(*FEMDocumentation/tutorial/SolvingPDEwithFEM*)
(CapitalOmega)=RegionDifference(Rectangle({-1,-1},{1,1}),Rectangle({-1/2,-1/2},{1/2,1/2}));
op={-Derivative(0, 2)(u)(x, y) - Derivative(2, 0)(u)(x,
y) - Derivative(1, 1)(v)(x, y),
-Derivative(1, 1)(u)(x, y) - Derivative(0, 2)(v)(x,
y) - Derivative(2, 0)(v)(x, y)}
Subscript((CapitalGamma), D)={DirichletCondition({u(x,y)==1.,v(x,y)==0.},x==1/2&&-1/2<=y<=1/2),DirichletCondition({u(x,y)==-1.,v(x,y)==0.},x==-1/2&&-1/2<=y<=1/2),DirichletCondition({u(x,y)==0.,v(x,y)==-1.},y==-1/2&&-1/2<=x<=1/2),DirichletCondition({u(x,y)==0.,v(x,y)==1.},y==1/2&&-1/2<=x<=1/2),DirichletCondition({u(x,y)==0.,v(x,y)==0.},Abs(x)==1||Abs(y)==1)}
{ufun,vfun}=NDSolveValue({op=={0,0},Subscript((CapitalGamma), D)},{u,v},{x,y}(Element)(CapitalOmega), StartingStepSize->0.1,MaxStepSize->0.01, WorkingPrecision->30,InterpolationOrder->All, NormFunction->(Norm(#, 1)&))
ContourPlot(ufun(x,y),{x,y}(Element)(CapitalOmega),ColorFunction->"Temperature",AspectRatio->Automatic,PlotPoints->30,WorkingPrecision->20,Contours->30)
```

But the ndsolve value of MMA cannot be used to solve finite element problems according to the voltage limit conditions

```
Clear("Gloabal`*")
(CapitalOmega) =
RegionDifference(Rectangle({-1, -1}, {1, 1}),
Rectangle({-1/2, -1/2}, {1/2, 1/2}));
op = {Derivative(1, 0)((Sigma)x)(x, y) +
Derivative(0, 1)((Tau)xy)(x, y),
Derivative(0, 1)((Sigma)y)(x, y) +
Derivative(1, 0)((Tau)xy)(x, y),
Derivative(2, 0)((Sigma)x)(x, y) +
Derivative(0, 2)((Sigma)y)(x, y) +
2*Derivative(1, 1)((Tau)xy)(x, y)}
(*(PartialD)Subscript((Sigma),xx)(x,y)/(PartialD)x+(PartialD)
Subscript((Tau),xy)(x,y)/(PartialD)y(Equal)0 (PartialD)Subscript(
(Sigma),yy)(x,y)/(PartialD)y+(PartialD)Subscript((Tau),xy)(x,y)/
(PartialD)x(Equal)0*)
Subscript((CapitalGamma),
D) = {DirichletCondition({(Sigma)x(x, y) == 10., (Sigma)y(x, y) ==
0., (Tau)xy(x, y) == 0.},
Abs(x) == 1/2 && -1/2 <= y <= 1/2 || -1/2 <= x <= 1/2 &&
Abs(y) == 1/2),
DirichletCondition({(Sigma)x(x, y) == -10., (Sigma)y(x, y) ==
0., (Tau)xy(x, y) == 0.}, Abs(x) == 1 || Abs(y) == 1)}
(*{ufun,vfun,wfun}=NDSolveValue({op(Equal){0,0,0},Subscript(
(CapitalGamma),D)},{(Sigma)x,(Sigma)y,(Tau)xy},{x,0,5},{y,0,1},
Method(Rule){"PDEDiscretization"(Rule){"MethodOfLines",{
"SpatialDiscretization"(Rule)"FiniteElement"}}})*)
{ufun, vfun, wfun} =
NDSolveValue({op == {0, 0, 0},
Subscript((CapitalGamma),
D)}, {(Sigma)x, (Sigma)y, (Tau)xy}, {x,
y} (Element) (CapitalOmega), StartingStepSize -> 0.1,
MaxStepSize -> 0.01, WorkingPrecision -> 20)
ContourPlot(ufun(x, y), {x, y} (Element) (CapitalOmega),
ColorFunction -> "Temperature", AspectRatio -> Automatic)
```

The result of this image is obviously incorrect.