}
    /// Create a new builder from a given set of lines.
    pub fn from_docs(docs: Vec<Doc>) -> Self {
        Self { docs }

#[cfg(feature = "parsers")]
mod parse {
    use nom::{
        bytes::complete::{is_not, tag, take},
        character::complete::{line_ending, not_line_ending, space1},
        combinator::{map, map_res, opt, peek},
        sequence::{pair, preceded, terminated},
        IResult,
    };
    use super::*;
    fn line(input: &str) -> IResult<&str, &str> {
        terminated(not_line_ending, line_ending)(input)
    }
    pub fn text(input: &str) -> IResult<&str, Doc> {
        map(line, |s: &str| {
            if s.is_empty() {
                Doc::Blank
            } else {
                Doc::Text(s.to_string())
            }
        })(input)
    }
    pub fn link(input: &str) -> IResult<&str, Doc> {
        let body = pair(is_not(" \t\r\n"), opt(preceded(space1, is_not(" \t\r\n"))));
        map(
            preceded(pair(tag("=>"), space1), terminated(body, line_ending)),
            |(to, name): (&str, Option<&str>)| Doc::Link {
                to: to.to_string(),
                name: name.map(|s| s.to_string()),
            },
        )(input)
    }
    fn level(input: &str) -> IResult<&str, Level> {
        map_res(peek(take(3usize)), |s| {
            Ok(match s {
                "###" => Level::Three,
                _ if s.starts_with("##") => Level::Two,
                _ if s.starts_with("#") => Level::One,
                _ => return Err(()),
            })
        })(input)
    }
    pub fn heading(input: &str) -> IResult<&str, Doc> {
        map(
            terminated(pair(level, not_line_ending), line_ending),
            |(lvl, s)| Doc::Heading(lvl, s.to_string()),
        )(input)
    }
    pub fn list_item(input: &str) -> IResult<&str, Doc> {
        todo!()
    }
    pub fn quote(input: &str) -> IResult<&str, Doc> {
        todo!()
    }
    pub fn preformatted(input: &str) -> IResult<&str, Doc> {
        todo!()
    }
    pub fn document(input: &str) -> IResult<&str, Builder> {
        todo!()
    }
}