Function command_kind 

pub fn command_kind<'i, Input, Error>(
    input: &mut Input,
) -> Result<CommandKind, Error>
where
    Input: StreamIsPartial + Stream + Compare<&'i str>,
    <Input as Stream>::Slice: AsBStr,
    <Input as Stream>::Token: AsChar + Clone,
    Error: ParserError<Input>,

Parse the command kind from the prefix keyword.

This parses count( or union( and returns the command kind.