General ⇒ Changing Em Dashes?

[rhk]

Since I use em-dashes much more often than I use en-dashes, and since when I write other things in plain text or html, I'm used to writing just "--" to get an em-dash, is there any way to customize LaTeX so that when I type "--" it turns into an em-dash instead of an en-dash? (This is the same sort of logic that has C use = for assigning values to variables and == for equality, since the former is used so much more.) I'm happy to type something more complicated on the rare occasion that I need an en-dash.