Currently, MultikeynessTrie uses opt::unordered_map, which is an alias for either std::unordered_map or stdx::unordered_map depending on a compile-time flag (kContainerImpl). One of these is failing to compile. The problem is that the map contains MultikeynessTrie by value: some but not all map types support this, because they use some pointer / indirection internally.

Let's make sure MultikeynessTrie compiles regardless of the value of kContainerImpl. One way is to have MultikeynessTrie use std::map directly instead of opt::unordered_map.