I’m foolish to procrastinate, must have been tired, this is easy to
show :

a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c
a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c
a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c a b c
a b c a b c a b c a b c a b c a b c

So between each a and b, and b and c, there is an unbreakable space, so
there should be breaks *only* between c and a.  `fill-paragraph' honours
that.

When looking the html version, which, modulo some extra time, majordomo
accepts to let pass, you will see linebreaks after a « a » or a « b ».