parameters: (self, key, value)
docstring: Adds a (name, value) pair, doesn't overwrite the value if it already
          exists.
  
          >>> headers = HTTPHeaderDict(foo='bar')
          >>> headers.add('Foo', 'baz')
          >>> headers['foo']
          'bar, baz'
docstring_summary: Adds a (name, value) pair, doesn't overwrite the value if it already
          exists.
docstring_tokens: Adds a ( name value ) pair doesn t overwrite the value if it already exists .
function: def add(self, key, value):
          """Adds a (name, value) pair, doesn't overwrite the value if it already
          exists.
  
          >>> headers = HTTPHeaderDict(foo='bar')
          >>> headers.add('Foo', 'baz')
          >>> headers['foo']
          'bar, baz'
          """
          self._data.setdefault(key.lower(), []).append((key, value))
function_tokens: def add ( self , key , value ) : self . _data . setdefault ( key . lower ( ) , [ ] ) . append ( ( key , value ) )