<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _parse_symbol(self, sym):
 """ Parse a string with a symbol to extract a string representing an element. Args: sym (str):
 A symbol to be parsed. Returns: A string with the parsed symbol. None if no parsing was possible. """

        # Common representations for elements/water in cif files
        # TODO: fix inconsistent handling of water
        special = {"Hw": "H", "Ow": "O", "Wat": "O",
                   "wat": "O", "OH": "", "OH2": "", "NO3": "N"}

        parsed_sym = None
        # try with special symbols, otherwise check the first two letters,
        # then the first letter alone. If everything fails try extracting the
        # first letters.
        m_sp = re.match("|".join(special.keys()), sym)
        if m_sp:
            parsed_sym = special[m_sp.group()]
        elif Element.is_valid_symbol(sym[:2].title()):
            parsed_sym = sym[:2].title()
        elif Element.is_valid_symbol(sym[0].upper()):
            parsed_sym = sym[0].upper()
        else:
            m = re.match(r"w?[A-Z][a-z]*", sym)
            if m:
                parsed_sym = m.group()

        if parsed_sym is not None and (m_sp or not re.match(r"{}\d*".format(parsed_sym), sym)):
            msg = "{} parsed as {}".format(sym, parsed_sym)
            warnings.warn(msg)
            self.errors.append(msg)

        return parsed_sym