In a recent question on Stack Overflow about speeding up group-by operations, Marek wondered which functions called .Internal code (and consequently were fast). I was surprised to see that there appears to be no built-in way to check whether or not this is the case (though is.primitive is available for primitive functions). Writing such a function is quite straight forward....
Zero Inflated Models and Generalized Linear Mixed Models with R.
Zuur, Saveliev, Ieno (2012).
