Classes in R

Two quick questions.

First, why doesn't using the matrix function actually set the class of the
resulting object to "matrix"?

a <- matrix(1,10,10)
attributes(a)

$dim
 [1] 10 10

or should I be using?

a <- matrix(1,10,10); class(a) <- c("matrix");
attributes(a)

$dim
[1] 10 10

$class
[1] "matrix"

Second, can somone please send me an example showing how inheritance works
in R?  The R Language Definition document isn't particularly clear on the
subject... i.e. NextMethod lacks a good example.

Cheers,
Randall

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