How to use the soupsieve.util.SelectorSyntaxError function in soupsieve

def process_custom(custom):
    """Process custom."""

    custom_selectors = {}
    if custom is not None:
        for key, value in custom.items():
            name = util.lower(key)
            if RE_CUSTOM.match(name) is None:
                raise SelectorSyntaxError("The name '{}' is not a valid custom pseudo-class name".format(name))
            if name in custom_selectors:
                raise KeyError("The custom selector '{}' has already been registered".format(name))
            custom_selectors[css_unescape(name)] = value
    return custom_selectors

if m is None:
                c = pattern[index]
                # If the character represents the start of one of the known selector types,
                # throw an exception mentioning that the known selector type is in error;
                # otherwise, report the invalid character.
                if c == '[':
                    msg = "Malformed attribute selector at position {}".format(index)
                elif c == '.':
                    msg = "Malformed class selector at position {}".format(index)
                elif c == '#':
                    msg = "Malformed id selector at position {}".format(index)
                elif c == ':':
                    msg = "Malformed pseudo-class selector at position {}".format(index)
                else:
                    msg = "Invalid character {!r} position {}".format(c, index)
                raise SelectorSyntaxError(msg, self.pattern, index)
        if self.debug:  # pragma: no cover
            print('## END PARSING')

has_selector, is_html = self.parse_pseudo_class(sel, m, has_selector, iselector, is_html)
                elif key == 'pseudo_element':
                    raise NotImplementedError("Pseudo-element found at position {}".format(m.start(0)))
                elif key == 'pseudo_contains':
                    has_selector = self.parse_pseudo_contains(sel, m, has_selector)
                elif key in ('pseudo_nth_type', 'pseudo_nth_child'):
                    has_selector = self.parse_pseudo_nth(sel, m, has_selector, iselector)
                elif key == 'pseudo_lang':
                    has_selector = self.parse_pseudo_lang(sel, m, has_selector)
                elif key == 'pseudo_dir':
                    has_selector = self.parse_pseudo_dir(sel, m, has_selector)
                    # Currently only supports HTML
                    is_html = True
                elif key == 'pseudo_close':
                    if not has_selector:
                        raise SelectorSyntaxError(
                            "Expected a selector at postion {}".format(m.start(0)),
                            self.pattern,
                            m.start(0)
                        )
                    if is_open:
                        closed = True
                        break
                    else:
                        raise SelectorSyntaxError(
                            "Unmatched pseudo-class close at postion {}".format(m.start(0)),
                            self.pattern,
                            m.start(0)
                        )
                elif key == 'combine':
                    if is_relative:
                        has_selector, sel, rel_type = self.parse_has_combinator(

if has_selector:
                        raise SelectorSyntaxError(
                            "Tag name found at position {} instead of at the start".format(m.start(0)),
                            self.pattern,
                            m.start(0)
                        )
                    has_selector = self.parse_tag_pattern(sel, m, has_selector)
                elif key in ('class', 'id'):
                    has_selector = self.parse_class_id(sel, m, has_selector)

                index = m.end(0)
        except StopIteration:
            pass

        if is_open and not closed:
            raise SelectorSyntaxError(
                "Unclosed pseudo-class at position {}".format(index),
                self.pattern,
                index
            )

        if has_selector:
            if not sel.tag and not is_pseudo:
                # Implied `*`
                sel.tag = ct.SelectorTag('*', None)
            if is_relative:
                sel.rel_type = rel_type
                selectors[-1].relations.append(sel)
            else:
                sel.relations.extend(relations)
                del relations[:]
                selectors.append(sel)

elif key == 'pseudo_dir':
                    has_selector = self.parse_pseudo_dir(sel, m, has_selector)
                    # Currently only supports HTML
                    is_html = True
                elif key == 'pseudo_close':
                    if not has_selector:
                        raise SelectorSyntaxError(
                            "Expected a selector at postion {}".format(m.start(0)),
                            self.pattern,
                            m.start(0)
                        )
                    if is_open:
                        closed = True
                        break
                    else:
                        raise SelectorSyntaxError(
                            "Unmatched pseudo-class close at postion {}".format(m.start(0)),
                            self.pattern,
                            m.start(0)
                        )
                elif key == 'combine':
                    if is_relative:
                        has_selector, sel, rel_type = self.parse_has_combinator(
                            sel, m, has_selector, selectors, rel_type, index
                        )
                    else:
                        has_selector, sel = self.parse_combinator(
                            sel, m, has_selector, selectors, relations, is_pseudo, index
                        )
                elif key == 'attribute':
                    has_selector = self.parse_attribute_selector(sel, m, has_selector)
                elif key == 'tag':

sel.rel_type = rel_type
            selectors[-1].relations.append(sel)
            rel_type = ":" + WS_COMBINATOR
            selectors.append(_Selector())
        else:
            if has_selector:
                # End the current selector and associate the leading combinator with this selector.
                sel.rel_type = rel_type
                selectors[-1].relations.append(sel)
            elif rel_type[1:] != WS_COMBINATOR:
                # It's impossible to have two whitespace combinators after each other as the patterns
                # will gobble up trailing whitespace. It is also impossible to have a whitespace
                # combinator after any other kind for the same reason. But we could have
                # multiple non-whitespace combinators. So if the current combinator is not a whitespace,
                # then we've hit the multiple combinator case, so we should fail.
                raise SelectorSyntaxError(
                    'The multiple combinators at position {}'.format(index),
                    self.pattern,
                    index
                )
            # Set the leading combinator for the next selector.
            rel_type = ':' + combinator
        sel = _Selector()

        has_selector = False
        return has_selector, sel, rel_type

def parse_pseudo_class_custom(self, sel, m, has_selector):
        """
        Parse custom pseudo class alias.

        Compile custom selectors as we need them. When compiling a custom selector,
        set it to `None` in the dictionary so we can avoid an infinite loop.
        """

        pseudo = util.lower(css_unescape(m.group('name')))
        selector = self.custom.get(pseudo)
        if selector is None:
            raise SelectorSyntaxError(
                "Undefined custom selector '{}' found at postion {}".format(pseudo, m.end(0)),
                self.pattern,
                m.end(0)
            )

        if not isinstance(selector, ct.SelectorList):
            self.custom[pseudo] = None
            selector = CSSParser(
                selector, custom=self.custom, flags=self.flags
            ).process_selectors(flags=FLG_PSEUDO)
            self.custom[pseudo] = selector

        sel.selectors.append(selector)
        has_selector = True
        return has_selector

def __init__(self, msg, pattern=None, index=None):
        """Initialize."""

        self.line = None
        self.col = None
        self.context = None

        if pattern is not None and index is not None:
            # Format pattern to show line and column position
            self.context, self.line, self.col = get_pattern_context(pattern, index)
            msg = '{}\n  line {}:\n{}'.format(msg, self.line, self.context)

        super(SelectorSyntaxError, self).__init__(msg)