'Return a pretty-printed string representation of this
chart\'s leaves.  This string can be used as a header
for calls to ``pretty_format_edge``.'

def pretty_format_leaves(self, width=None):

    if (width is None):
        width = (50 // (self.num_leaves() + 1))
    if ((self._tokens is not None) and (width > 1)):
        header = u'|.'
        for tok in self._tokens:
            header += (tok[:(width - 1)].center((width - 1)) + u'.')
        header += u'|'
    else:
        header = u''
    return header
