Suggest a plausible mechanism for the formation of the following quaternary ammonium salt.

Stepwise mechanism showing the formation of a quaternary ammonium salt from amine and CH3Br.

Detailed mechanism for quaternary ammonium salt formation using HCO3 and CH3Br.

Comprehensive mechanism illustrating quaternary ammonium salt formation with multiple steps.

Complex mechanism for quaternary ammonium salt formation involving HCO3 and CH3Br.