Bindings and wrapper for the Bullet Physics C API.
Unfortunately Bullet's C API is extremely limited, and is not regularly updated. The header has this message at the top:
Draft high-level generic physics C-API. For low-level access, use the physics SDK native API's. Work in progress, functionality will be added on demand. If possible, use the richer Bullet C++ API, by including btBulletDynamicsCommon.h"
Which doesn't really help folks who work in other languages! In the future it might be better to implement a custom C API in order to provide a more comprehensive wrapper for Bullet.