Trying to learn how to write an "advanced" function

The first thing to understand is that despite similarity in names,
`match` and `match.call` are doing very different things, which should
not be confused with each other.

For understanding what a function is doing, it is helpful to watch
what it does at each step.  With functions like `lm` that are built
in, we cannot easily modify the source code with print statements and
the like, then rerun, but the `trace` and `browser` functions are nice
for allowing us to debug these functions. If you run the following
line:

trace(lm, browser, at=7)

Then this will insert a call to `browser` into the `lm` function which
when you next run `lm` will pause at about line 7 (just after the code
that you show) and allow you to examine the values of `mf` and `m` and
any other variables.

Unfortunately just printing `mf` is not super helpful for this, since
it prettys the call, but printing as.list(mf) is more useful.
Basically mf will hold information on how `lm` was called, the first
element is that `lm` is the function that was called, then further
elements are the arguments that were passed in.

The line with `match` then identifies which elements of mf correspond
to the list of arguments that we want to keep for the next part
(essentially the next few lines create a call object to the
`model.frame` funcion with the same arguments that you passed to `lm`
but without any arguments not in the short list.

When in `browser` mode you can step through the evaluation of the
function with "s" or "n" and quit everything with "Q" (see ?browser
for details)

Don't forget to call `untrace(lm)` when you are through.

I am trying to understand how to write an "advanced" function. To do so, I am examining the lm fucnction, a portion of which is pasted below. I am unable to understand what match.call or   match does, and several other parts of lm, even when I read the help page for match.call or match.
(1) can someone point me to an explanation of match.call or match that can be understood by the uninitiated?
(2) can someone point me to a document that will help me learn how to write an "advanced" function?

Thank you,
John

lm

function (formula, data, subset, weights, na.action, method = "qr",
    model = TRUE, x = FALSE, y = FALSE, qr = TRUE, singular.ok = TRUE,
    contrasts = NULL, offset, ...)
{
    ret.x <- x
    ret.y <- y
    cl <- match.call()
    mf <- match.call(expand.dots = FALSE)
    m <- match(c("formula", "data", "subset", "weights", "na.action",
        "offset"), names(mf), 0L)

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