Reduction function with given operation.

    Args:
        x (Variable): An input.
        op (str): 'sum' or 'mean'.

    Note:
        This is deprecated. Use ``mean`` or ``sum`` instead.

def reduce(x, op='sum'):
    """Reduction function with given operation.

    Args:
        x (Variable): An input.
        op (str): 'sum' or 'mean'.

    Note:
        This is deprecated. Use ``mean`` or ``sum`` instead.

    """
    import warnings
    warnings.warn(
        "Deprecated API. Use ``sum`` or ``mean`` instead.", DeprecationWarning)
    from .function_bases import reduce_sum, reduce_mean
    if op == 'sum':
        return reduce_sum(x)
    elif op == 'mean':
        return reduce_mean(x)
    raise ValueError()