docs: def valid(self, instance, schema):
          """Validate schema."""
          try:
              jsonschema.validate(instance, schema)
          except jsonschema.exceptions.ValidationError as ex:
              self.stderr.write("    VALIDATION ERROR: {}".format(instance['name'] if 'name' in instance else ''))
              self.stderr.write("        path:       {}".format(ex.path))
              self.stderr.write("        message:    {}".format(ex.message))
              self.stderr.write("        validator:  {}".format(ex.validator))
              self.stderr.write("        val. value: {}".format(ex.validator_value))
              return False
  
          try:
              # Check that default values fit field schema.
              for field in ['input', 'output', 'schema']:
                  for schema, _, path in iterate_schema({}, instance.get(field, {})):
                      if 'default' in schema:
                          validate_schema({schema['name']: schema['default']}, [schema])
          except ValidationError:
              self.stderr.write("    VALIDATION ERROR: {}".format(instance['name']))
              self.stderr.write("        Default value of field '{}' is not valid.". format(path))
              return False
  
          return True