LaTeX: Nested optional arguments

Latex allows you to make commands with an optional argument.  For example,


\newcommand{\Line}[1][G]{\overline{{#1}}}

Now $\Line$ will draw a G with a line above it, and $\Line[H]$ will draw

an H with a line above.  Nesting these can cause problems though.  For instance,

$\Line[\Line]$ works, but $\Line[\Line[H]]$ doesn't.  The solution is to always

wrap the optional argument in an extra set of curly braces: $\Line[{\Line[H]}]$.