[Zope] start level of title tags in structured text

how can we tweak the start level of the title tags in structured text: so that when a structured text, written by an internaut for example, is integrated into a page, it doesn't break the structrure html of the rendered page in the case of plone, the rendered html can contain 2 <h1> as the description of the page has already the tag <h1> and these two <h1> are not necessarily on the same "logical" level. I can't see any way to start the contents of the stx-text with, for example a title of level 2 my google research was not fructful.... thanks