[R] Extracting formulae from expression() / deriv()

[nickymcp]

I am attempting to extract the derivative/ gradient from this expression 

df1p <- deriv(f1, "P")
> df1p
expression({
    .value <- s - c - a * P
    .grad <- array(0, c(length(.value), 1L), list(NULL, c("P")))
    .grad[, "P"] <- -a
    attr(.value, "gradient") <- .grad
    .value
})

So in this case I want to extract the "-a".

I have read the expression help file, which tells me that I should use [ and
[[ to extract information, but the best result I've had so far is 

> df1p[attr(f1,"gradient")]
expression()

I'm sure I'm missing something really basic, but I've now lost several hours
trying to do this and I'm getting pretty desperate.
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