I'm having trouble installing Pyfst, which is a Python wrapper for OpenFst. I've tried using

pip install pyfst    

but always end up with an error about some "unordered_map" which can't be found. Here's part of the stack trace:

In file included from fst/_fst.cpp:321:

/usr/local/include/fst/symbol-table.h:26:10: fatal error: 'unordered_map' file not found

#include <unordered_map>