A silly LaTeX error took me a few minutes too many to solve: I defined
which got me the error message
TeX capacity exceeded , sorry [ grouping levels =255].
that I understood as a recursive definition. So I instead pre-defined the new θ as
which did not work either… After google-ing the issue, I found this on-line LaTeX Wikibook that provided me with the solution:
which worked. Of course, a global change of \theta into \btheta would have been much much faster to execute….