code: public StructuredQueryDefinition containerConstraint(String constraintName, StructuredQueryDefinition query) {
      checkQuery(query);
      return new ContainerConstraintQuery(constraintName, query);
    }
docs: Matches a query within the substructure of the container specified
  by the constraint.
  @param constraintName    the constraint definition
  @param query    the query definition
  @return    the StructuredQueryDefinition for the element constraint query