'Return a newly added `w:style` element having *name* and
*style_type*. `w:style/@customStyle` is set based on the value of
*builtin*.'

def add_style_of_type(self, name, style_type, builtin):

    style = self.add_style()
    style.type = style_type
    style.customStyle = (None if builtin else True)
    style.styleId = styleId_from_name(name)
    style.name_val = name
    return style
