private function _getXAdvancePos($pos)
	{
		// NB Not all fonts have all marks specified in GlyphClassMarks
		// If the current glyph is not a base (but a mark) then ignore this, and apply to the current position
		if (strpos($this->GlyphClassMarks, $this->OTLdata[$pos]['hex']) !== false) {
			return $pos;
		}

		while (isset($this->OTLdata[$pos + 1]['hex']) && strpos($this->GlyphClassMarks, $this->OTLdata[$pos + 1]['hex']) !== false) {
			$pos++;
		}
		return $pos;
	}