Math/rand not thread safe

[steve-hb]

When generating a few million sentences with 16 threads I noticed sudden crashes after few seconds. The reason behind is the use of the math/rand package in a non-synchronized manner.

Currently I don't have the time to fix this bug unfortunately, maybe later.

Workaround could be to generate them in one thread and push them into a channel for later use. (that's what I do for one of my tests)