code: public static Mapping<Boolean> bool(Constraint... constraints) {
          return new FieldMapping(
                  InputMode.SINGLE,
                  mkSimpleConverter(s ->
                      isEmptyStr(s) ? false : Boolean.parseBoolean(s)
                  ), new MappingMeta(MAPPING_BOOLEAN, Boolean.class)
              ).constraint(checking(Boolean::parseBoolean, "error.boolean", true))
                  .constraint(constraints);
          }
docs: (convert to Boolean) mapping
  @param constraints constraints
  @return new created mapping