Hypatia is a pure-C math library. It is almost 100% C89/C90 compliant. This library is intended for use in 2d/3d graphics program (such as games). Since it is not a general purpose math library, but a library meant for 3d graphics, certain opinions have been expressed in its design. One of those design choices, intended to help with speed, is that all objects (quaternions, matrices, vectors) are mutable. (That means that the objects change their values.) This was a purposeful design choice. Construct your program around this choice.
This is a companion discussion topic for the original entry at https://copr.fedorainfracloud.org/coprs/dagostinelli/hypatia/