Potential nontermination for flat operators

The clash between flattening rules and flat pattern matching causes nontermination with some flat operators. For example:

```
f(f(a, f(b, c), d), f(e, f(g, h)))
```

This exists regardless of whether the flat operator is explicitly given in the rule (e.g. current `master`) or if the operator is variable (e.g. #52).

Start by reproducing the example term in the project and tracing the interaction between flattening rules and flat pattern matching. Done means the example and equivalent cases terminate reliably, with behavior covering both explicitly named and variable flat operators.

Written by the indexing model from the issue text.