def _resolve_prefix(celt, prefix):
namespace = None<EOL>while _is_element(celt):<EOL><INDENT>if prefix:<EOL><INDENT>namespaceURI = _find_xmlns_prefix(celt, prefix)<EOL><DEDENT>else:<EOL><INDENT>namespaceURI = _find_default_namespace(celt)<EOL><DEDENT>if namespaceURI: break<EOL>celt = celt.parentNode<EOL><DEDENT>else:<EOL><INDENT>if prefix:  <EOL><INDENT>raise EvaluateException('<STR_LIT>' %prefix)<EOL><DEDENT><DEDENT>return namespaceURI<EOL>
resolve prefix to a namespaceURI.  If None or 
    empty str, return default namespace or None.

    Parameters:
      celt -- element node
      prefix -- xmlns:prefix, or empty str or None