code: public void registerMetadataExpression(String expression, int targetDepth, String storageKey) {
          metadataExpressions.add(new MetadataExpression(expression, targetDepth, storageKey));
      }
docs: Registers an expression, which if matched, will cause the data for the
  matching element to be stored in the metadata map under the specified
  key.
  
  @param expression
  The expression an element must match in order for it's data to
  be pulled out and stored in the metadata map.
  @param targetDepth
  The depth in the XML document where the expression match must
  start.
  @param storageKey
  The key under which to store the matching element's data.