Speedier Erlang

In the comments to Erlang Fractal Benchmark, Ulf Wiger noted in the comments that adding is_float guard clauses speeds up the code.

When I asked why on the Erlang email list, Ulf and others explained: when the compiler sees is_float(X) it knows that X must be a float. Instead of worrying about type checks, casting, and other inefficiencies, it can optimize the code that uses X.