I tried to substitute the delta symbol instead of the product of metrics, but the expression in the end somehow equals 0. Am I doing something wrong or is this a bug?

```
{a,b,c,d,e,f,g,h,i,j,k,l,m,n,q,r,s,u,v,w,z#}::Indices(position=independent).
h_{m n}::Metric.
h^{m n}::InverseMetric.
\delta^{m?}_{n?}::KroneckerDelta.
\delta_{m?}^{n?}::KroneckerDelta.
{\partial{#}}::PartialDerivative.
{\nabla{#},\delta{#}}::Derivative.
delh_{m n}::Depends(\nabla{#}, \partial{#}).
delh^{m n}::Depends(\nabla{#}, \partial{#}).
h_{m n}::Depends(\partial{#}, \delta{#}).
h^{m n}::Depends(\partial{#},\delta{#}).
\Dh::LaTeXForm("\sqrt{-h}").
delh{#}::LaTeXForm("\delta h").
ex:= \nabla_{a}{\nabla_{b}{delh^{c a}}}h_{c d} h^{d b};
substitute(ex, $h_{m1 m2} h^{m2 m3} -> \delta_{m1}^{m3}$);
```