creating function bodies using body()

I'm having trouble figuring out how to create a function using "body<-"
().  The help file for body() says that the argument should be a list of
R expressions.  However if I try that I get an error:

tmpfun <- function(a, b=2){}
body(tmpfun) <- list(expression(z <- a + b),expression(z^2))

Error in as.function.default(c(formals(f), value), envir) :
        invalid formal argument list for "function"

Can someone give me a simple example for doing this?  Thanks!

tmpfun <- function(a, b=2){}
body(tmpfun) <- quote({z <- a + b; z^2})
tmpfun(3)