expressions (nesting, substitution, 2-stage evaluation)

Hi,

I have a function which generates many plots.  To keep it simple, let's say
I want to set the main title based on where we are in nested loops.

So, something like

vectorA = c("a","b","c")
vectorB = c("a","b","c")

for(ii in vectorA) { for(jj in vectorB) {
  plot(0:1,0:1)
  title(main = paste(ii,jj))
}

that part is easy!   The question is what if I wanted vectorA to be an
expression?

I'd like to be able to set vectorA =
c(expression(paste("TNF-",alpha)),expression(paste("IFN-",gamma))), and have
the plot title show the greek letters.

Obviously, in the for-loop I could build the expression all at once, but
there are lots of programmatic reasons I'd like to be able to have this
program structure.  Is there a solution which modifies either/both (1) the
setting of main in the loop (2) how I define the vector outside of the loop?


thanks, Daryl