Construct a function with argments specified by an R expression

I want to build a function (say "buildFn") to *return* a function of the
form

    foo(x,y,a1, ... an, b1, ..., bn)

where the arguments ai and bi are given in the form of a list created
in R.  E.g. I'd like to be able to say

     argnms <- c(paste0("a",1:3),paste0("b",1:3"))
     foo <- buildFn(argnms)

with the resulting "foo" having arguments x, y, a1, a2, a3, b1, b2, b3.

I thought I might be able to do this using "formals<-" but I cannot get
this to work.  i could provide more detail of what I've tried, but it's
probably not worth it.

Can anyone steer me in the right direction?  Thanks.

cheers,

Rolf Turner

-- 
Honorary Research Fellow
Department of Statistics
University of Auckland
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