'Adds the help entry.
:param content: usually a tuple like (head, body)
:param cat: a name of the category.
If the item exists in an other category, it will be replaced.'

def add_help_entry(self, subj, content, cat=''):

    if (type(content) not in (tuple, list)):
        content = (content, None)
    self._table[subj] = content
    self._subj2Gat[subj] = cat
    if (cat in self._cat2Subj):
        d = self._cat2Subj[cat]
    else:
        d = []
        self._cat2Subj[cat] = d
    d.append(subj)
