extension PureYAML.Parsing.Scanner {
    func scanNamedToken(
        kind: (String) -> PureYAML.Parsing.TokenKind,
        missingNameError: (Int) -> PureYAML.Parsing.ParseError,
        state: inout State,
    ) throws {
        let start = state.reader.mark
        // An anchor or alias may itself begin a flow simple key, so record the
        // position before the token is appended; the following node then does not
        // start a new key, leaving `&anchor key :` anchored at the `&` (libyaml's
        // fetch_anchor saves the simple key before clearing simple_key_allowed).
        saveFlowSimpleKey(&state)
        state.reader.advance()
        // An anchor or alias name is a run of name characters that ends at the
        // first non-name character, mirroring `yaml_parser_scan_anchor`: the
        // name must stop at whitespace, `:`, `?`, flow indicators, and the other
        // indicator characters so that `*alias:` reads the alias `alias`
        // followed by a `:` mapping indicator rather than swallowing the colon.
        let name = state.reader.consume { character in
            isAnchorNameCharacter(character)
        }
        guard !name.isEmpty else {
            throw missingNameError(start.line)
        }
        state.append(kind(name), mark: start, endMark: state.reader.mark)
    }

    func scanTag(_ state: inout State) throws {
        let start = state.reader.mark
        // A tag may itself begin a flow simple key (see `scanNamedToken`).
        saveFlowSimpleKey(&state)
        state.reader.advance()
        var value = "!"
        if state.reader.peek() == "<" {
            value.append(state.reader.advance() ?? "<")
            var didClose = false
            while let character = state.reader.peek() {
                value.append(character)
                state.reader.advance()
                if character == ">" {
                    didClose = true
                    break
                }
            }
            guard didClose else {
                throw PureYAML.Parsing.ParseError.unterminatedTag(line: start.line)
            }
        } else {
            value += state.reader.consume { character in
                !isTokenTerminator(character)
            }
            value = expandTag(value, state: state)
        }
        state.append(.tag(value), mark: start, endMark: state.reader.mark)
    }
}