code: def set_default (feature, value):
      
      f = __all_features[feature]
      bad_attribute = None
  
      if f.free:
          bad_attribute = "free"
      elif f.optional:
          bad_attribute = "optional"
  
      if bad_attribute:
          raise InvalidValue ("%s property %s cannot have a default" % (bad_attribute, f.name))
  
      if value not in f.values:
          raise InvalidValue ("The specified default value, '%s' is invalid.\n" % value + "allowed values are: %s" % f.values)
  
      f.set_default(value)
docs: Sets the default value of the given feature, overriding any previous default.
          feature: the name of the feature
          value: the default value to assign