<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def makeMissingRequiredGlyphs(font, glyphSet):
 """ Add .notdef to the glyph set if it is not present. **This should not be called externally.** Subclasses may override this method to handle the glyph creation in a different way if desired. """

        if ".notdef" in glyphSet:
            return

        unitsPerEm = otRound(getAttrWithFallback(font.info, "unitsPerEm"))
        ascender = otRound(getAttrWithFallback(font.info, "ascender"))
        descender = otRound(getAttrWithFallback(font.info, "descender"))
        defaultWidth = otRound(unitsPerEm * 0.5)
        glyphSet[".notdef"] = StubGlyph(name=".notdef",
                                        width=defaultWidth,
                                        unitsPerEm=unitsPerEm,
                                        ascender=ascender,
                                        descender=descender)