pub fn parse_partial_repl_line<'ast, Id, S>(
    arena: ArenaRef<'_, 'ast, Id>,
    symbols: &mut dyn IdentEnv<Ident = Id>,
    input: &S
) -> Result<Option<ReplLine<'ast, Id>>, (Option<ReplLine<'ast, Id>>, ParseErrors)>
where Id: Clone + Eq + Hash + AsRef<str> + Debug, S: ?Sized + ParserSource,