code: private void verifyJunctionField(FieldDefinition xlinkDef) {
          String juncField = xlinkDef.getXLinkJunction();
          if (!"_ID".equals(juncField)) {
              FieldDefinition juncFieldDef = m_fieldDefMap.get(juncField);
              Utils.require(juncFieldDef != null,
                            String.format("Junction field for xlink '%s' has not been defined: %s",
                                          xlinkDef.getName(), xlinkDef.getXLinkJunction()));
              Utils.require(juncFieldDef.getType() == FieldType.TEXT,
                            String.format("Junction field for xlink '%s' must be a text field: ",
                                          xlinkDef.getName(), xlinkDef.getXLinkJunction()));
          }
      }
docs: Verify that the given xlink's junction field is either _ID or an SV text field.