pub fn string(&self) -> Result<&String, SexpError> {
        match *self {
            Sexp::String(ref s) => Ok(s),
            _ => Err(format!("not a string: {}", self).into()),
        }
    }
/// access the symbolic-expression as if it is an `&String`
access the symbolic-expression as if it is an `&String`
access the symbolic - expression as if it is an ` &String `
pub fn string(&self) -> Result<&String, SexpError> {
        match *self {
            Sexp::String(ref s) => Ok(s),
            _ => Err(format!("not a string: {}", self).into()),
        }
    }
pub fn string ( & self ) -> Result < & String , SexpError > { match * self { Sexp :: String ( ref s ) => Ok ( s ) , _ => Err ( format ! ( " " self ) . into ( ) ) , } }
access the symbolic-expression as if it is an `&String`
access the symbolic - expression as if it is an ` &String `
&self