plotting mathematical notation and values substitution

Dear R-users,

I'm trying to add a title on a plot with both mathematical notation and
values substitution. I read the documentation and search the mailing list
but I was not able to solve my problem. Actually, there is a message by
Uwe Ligges on June 2003 which addresses a question very close to mine, but
the code provided doesn't work. The code is the following:

# I add this to let you run the example by copy and paste

t1 <- 0.5; len <- 1

# then

plot(1:10,

       main = substitute("Monotonic Multigamma run (" * n == len * ", " *
theta == t1 * ").", list(len = len, t1 = t1)))

but I got the following:

Error: syntax error

I also tried with just one value substitution:

plot(1:10,

       main = substitute("Monotonic Multigamma run (" theta == t1 * ").",
list(len = len, t1 = t1)))

which works fine. How can I have more than one value substitution,
together with mathematical notation and text?

Thanks in advance for any reply.

Luca Scrucca