The whole of mu4e consists of hundreds of elisp functions. However,
the majority of those are for internal use only; you can
recognize them easily, because they all start with mu4e~ or
mu4e--. These functions make all kinds of assumptions, and they
are subject to change, and should therefore not be used. The same
is true for variables with the same prefix; don’t touch them. Let
me repeat that:
Do not use mu4e~... or mu4e-- functions or variables!
In addition, you should use functions in the right context; functions
that start with mu4e-view- are only applicable to the message view,
while functions starting with mu4e-headers- are only applicable to
the headers view. Functions without such prefixes are applicable
everywhere.