code: public final String getExplicitPrefix(String uri)
      {
          if (mNsMapping != null) {
              String prefix = mNsMapping.findPrefixByUri(uri);
              if (prefix != null) {
                  return prefix;
              }
          }
          if (mRootNsContext != null) {
              String prefix = mRootNsContext.getPrefix(uri);
              if (prefix != null) {
                  // Hmmh... still can't use the default NS:
                  if (prefix.length() > 0) {
                      return prefix;
                  }
                  // ... should we try to find an explicit one?
              }
          }
          return null;
      }
docs: Method similar to {@link #getPrefix}, but one that will not accept
  the default namespace, only an explicit one. Usually used when
  trying to find a prefix for attributes.