code: def setnil(self, flag=True):
          
          p, u = Namespace.xsins
          name = ":".join((p, "nil"))
          self.set(name, str(flag).lower())
          self.addPrefix(p, u)
          if flag:
              self.text = None
          return self
docs: Set this node to I{nil} as defined by having an I{xsi:nil}=I{flag}
          attribute.
  
          @param flag: A flag indicating how I{xsi:nil} will be set.
          @type flag: boolean
          @return: self
          @rtype: L{Element}