Omit rules if they would be overridden by other rules (which are not yet triggered)

[davidpomerenke]

For example, don't replace ph with πh — it would be replaced by φ once the according rule triggers.

This should be done automatically and without any hacking in the rule files required. Actually, some hacking which is already going on there might turn out to be unnecessary once this is implemented.

[davidpomerenke]

Would be resolved by #8, so will wait to see whether implementing #8 is feasible.