def identity(x):
    """Temporarily copied from htools.

    Returns the input argument. Sometimes it is convenient to have this if
    we sometimes apply a function to an item: rather than defining a None
    variable, sometimes setting it to a function, then checking if it's None
    every time we're about to call it, we can set the default as identity and
    safely call it without checking.
    Parameters
    ----------
    x: any
    Returns
    -------
    x: Unchanged input.
    """
    return x
