code: static Claim extractClaim(String claimName, Map<String, JsonNode> tree, ObjectReader objectReader) {
          JsonNode node = tree.get(claimName);
          return claimFromNode(node, objectReader);
      }
docs: Helper method to extract a Claim from the given JsonNode tree.
  
  @param claimName the Claim to search for.
  @param tree      the JsonNode tree to search the Claim in.
  @return a valid non-null Claim.