numeric vector with predictions, corresponding to the
quantiles in a second vector, `quantiles`.
quantiles
numeric vector that denotes the quantile for the values
in `predictions`. Only those predictions where `quantiles == 0.5` will
be kept. If `quantiles` is `NULL`, then all `predictions` and
`true_values` will be used (this is then the same as `absolute_error()`)
verbose
logical, return a warning is something unexpected happens