We provide convenient extensions of the [dpq]gamma functions, which allow
the distribution to be specified in terms of its mean and standard deviation,
instead of shape and rate/scale.
qgamma(
  p,
  shape,
  rate = 1,
  scale = 1/rate,
  lower.tail = TRUE,
  log.p = FALSE,
  mean,
  sd
)dgamma(x, shape, rate = 1, scale = 1/rate, log = FALSE, mean, sd)
pgamma(
  q,
  shape,
  rate = 1,
  scale = 1/rate,
  lower.tail = TRUE,
  log.p = FALSE,
  mean,
  sd
)
vector of probabilities
see stats::GammaDist
mean and standard deviation, overriding shape and
rate or scale if specified
vector of quantiles