def constf(x):
  """
  Returns a constant function.
  
  Suppose 
    `f` = :code:`constf(x)`.
  
  Then, if `f` is applied with any `y`, it returns `x`. 
  
  Examples
  --------
  
  >>> f = constf(10)  
  >>> f([])
  10
  >>> f(3)
  10
  
  """
  return lambda _: x