Function one_of 

pub fn one_of(choices: &[&str], aliases: &[MatchArgAlias]) -> String

one of "fast", "slow": what a match_arg / choices value must be, in the words of a conversion’s argument error that names it beside other accepted values ('x' must be one of "oral", "bolus", or a single double: got logical, #1594). The listed aliases follow the choices (one of "fast", "slow" (or "quick" for "fast")).