I tried to calculate the d'Alembert operator in the case of the Friedman metric, and I had a kernel crash:

```
{a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,q,r,s,u,v,w,x,y,z#}::Indices(full,values={t,x,y}, position=independent).
{t, x, y}::Coordinate.
\partial{#}::PartialDerivative.
h_{m? n?}::Metric.
h^{m? n?}::InverseMetric.
h_{m? n?}::Symmetric.
h^{m? n?}::Symmetric.
h::Determinant(h_{m n}).
\Gamma^{m}_{n q}::TableauSymmetry(shape={2}, indices={1,2}).
p::Depends(t,x,y).
h_{m n}::Depends(t,x,y).
h^{m n}::Depends(t,x,y).
{a,H}::Depends(t).
friedmann:= { h_{t t} = 1,
h_{x x} = -a,
h_{y y} = -a,
}.
complete(friedmann, $h^{m n}$);
complete(friedmann, $h$);
Chr:= \Gamma^{l}_{m n} =
(1/2) * h^{l k} (
\partial_{n}{ h_{k m} } + \partial_{m}{ h_{k n} } - \partial_{k}{ h_{m n} } );
evaluate(Chr, friedmann, rhsonly=True)
substitute(Chr, $\partial_{t}{a} a**(-1) -> H$);
box := h^{a b}(\partial_{a b}{p} - \Gamma^{y}_{a b}\partial_{y}{p});
substitute(box,Chr);
evaluate(box, friedmann);
```

I'm using Cadabra 2.3.6.5 for mint Ulyana 20 in virtualbox and the same version for ubuntu 18.04 in WSL. There is an error from terminal:

```
Perm::apply: orig.size()=3, perm.size()=2
cadabra-server: /home/arshtm/cadabra2/core/./Permutations.hh:88: void Perm::apply(iterator, iterator) [with iterator = tree<cadabra::str_node>::sibling_iterator]: Assertion `orig.size()==perm.size()' failed.
```