Extracting Coefficients and Such from mle2 Output

The mle2 function (bbmle library) gives an example something like the
following in its help page. How do I access the coefficients, standard
errors, etc in the summary of "a"?

x <- 0:10
y <- c(26, 17, 13, 12, 20, 5, 9, 8, 5, 4, 8)
LL <- function(ymax=15, xhalf=6)

+   -sum(stats::dpois(y, lambda=ymax/(1+x/xhalf), log=TRUE))

a <- mle2(LL, fixed=list(xhalf=6))
summary(a)

Maximum likelihood estimation

Call:
mle2(minuslogl = LL, fixed = list(xhalf = 6))

Coefficients:
     Estimate Std. Error z value     Pr(z)    
ymax  19.2881     1.7115  11.269 < 2.2e-16 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 

-2 log L: 60.39244 


Tom

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