'Update (redefine) a prefix mapping for the branch.
@param p: A prefix.
@type p: basestring
@param u: A namespace URI.
@type u: basestring
@return: self
@rtype: L{Element}
@note: This method traverses down the entire branch!'

def updatePrefix(self, p, u):

    if (p in self.nsprefixes):
        self.nsprefixes[p] = u
    for c in self.children:
        c.updatePrefix(p, u)
    return self
