Builds a set of all field keys used in the pattern including nested fields.

        :param pattern: The kmatch pattern to get field keys from or None to use self.pattern
        :type pattern: list or None
        :returns: A set object of all field keys used in the pattern
        :rtype: set

def get_field_keys(self, pattern=None):
        """
        Builds a set of all field keys used in the pattern including nested fields.

        :param pattern: The kmatch pattern to get field keys from or None to use self.pattern
        :type pattern: list or None
        :returns: A set object of all field keys used in the pattern
        :rtype: set
        """
        # Use own pattern or passed in argument for recursion
        pattern = pattern or self.pattern

        # Validate the pattern so we can make assumptions about the data
        self._validate(pattern)

        keys = set()

        # Valid pattern length can only be 2 or 3
        # With key filters, field key is second item just like 3 item patterns
        if len(pattern) == 2 and pattern[0] not in self._KEY_FILTER_MAP:
            if pattern[0] in ('&', '|', '^'):
                # Pass each nested pattern to get_field_keys
                for filter_item in pattern[1]:
                    keys = keys.union(self.get_field_keys(filter_item))
            else:
                # pattern[0] == '!'
                keys = keys.union(self.get_field_keys(pattern[1]))
        else:
            # Pattern length is 3
            keys.add(pattern[1])
        return keys