[R] Simple way to define a function to be used in a formula object inside another function

[Frank Harrell]

I would like to do this:

f <- function(formula, data=NULL) {
  gg <- sqrt
  model.frame(formula, data=data)
  }
x <- y <- 1:10
f(y ~ gg(x))
Error in eval(expr, envir, enclos) : could not find function "gg"

Is there a simple way to get access to gg from within the model.frame 
invocation inside f?

Thanks
Frank