code: def forValue(klass, key, value, description):
          
  
          def validate(checked):
              if checked != value:
                  raise ValidationError(
                      checked, "Field %r must be %r" % (key, value))
  
          return klass(key, lambda _: value, description, validate)
docs: Create a L{Field} that can only have a single value.
  
          @param key: The name of the field, the key which refers to it,
              e.g. C{"path"}.
  
          @param value: The allowed value for the field.
  
          @param description: A description of what this field contains.
          @type description: C{unicode}
  
          @return: A L{Field}.