code: private void addNestedField(FieldDefinition nestedFieldDef) {
          // Prerequisites
          assert nestedFieldDef != null;
          assert m_type == FieldType.GROUP;
          Utils.require(!m_nestedFieldMap.containsKey(nestedFieldDef.getName()),
                        "Duplicate nested field name: %s", nestedFieldDef.getName());
          
          // Add it to us and point it at us.
          m_nestedFieldMap.put(nestedFieldDef.getName(), nestedFieldDef);
          nestedFieldDef.m_parentField = this;
      }
docs: Add the given FieldDefiniton as a child (nested field) of this field.