begin_x[1] is not the second component of the element, but the element
after the one pointed to begin_x. When begin_x points to the last
element, that line overwrites data past the end of the allocation, which,
during my tests, happened to contain the reference count for the matrix.
Hilarity ensues.

Looks like we don't actually use them; when creating precompiled headers
with Visual C++, a dummy .cpp is created at build time.