Helsinki Finite-State Technology Code

Add builting function Interpolate()

Fixed condition for including symbols in exclusionary lists
Rather than the printability vector, which may not be completely
set yet, check the symbol itself for printability.

Fix bug involving stale caches of function call results

Change python3 into python in README as version 2 works now too.

Move tools/src/hfst-string-conversions.[cc|h] under libhfst/src/.

Update version number to 3.9.1.

Do something useful in the case of a missing root rule

Make Makefiles in openfst back-end licensed as LGPL.

Candidate for release 3.9.1.

Fix names of files whose names were changed.