R help - Feb 2005 - D(eval(g)) problem, since "Function `eval' is not in the derivatives table"

thanks Andy and Dimitris for your reply to my expression/eval - problem
starting with the resulting expression g I need g's derivative as 
expression, but I get: "Function `eval' is not in the derivatives
table":

#function (double gamama)
h <- expression((t/d1)^a1*exp(-(t-d1)/b1)-c*(t/d2)^a2*exp(-(t-d2)/b2))
# plot it
t <- seq(0, 20000, by = 100)
t <- t/1000
g <- expression(eval(h)/sqrt(sum(eval(h)^2)))
plot(eval(g), type = 'l')
g.deriv <- D(g, "t")

 > Error in D(g, "t") : Function `eval' is not in the
derivatives table

is there any way one can solve this problem?

thanks a lot
christoph



## --
## parameters
peak1 <- 5.4
fwhm1 <- 5.2
peak2 <- 10.8
fwhm2 <- 7.35
dip <- 0.35

b1 <- 0.9 # dispersion
b2 <- 0.9 #dispersion
a1 <- peak1/b1
a2 <- peak2/b2
d1 <- a1*b1
d2 <- a2*b2
c <- dip