In the beamer manual on pg 64 they introduce the following suggestion on how to insert a horizontal navigation bar
\insertsectionnavigationhorizontal{.5\textwidth}{\hskip0pt plus1filll}{}
They introduce a variable, plus1filll, but I can't find information on where this comes from anywhere. I'd like to know: Is it a built-in LaTeX command or introduced by beamer? What does it do?
