{{alias}}( x, y, done )
    If a function does not return an error, invokes a callback with the function
    result; otherwise, invokes a second function `y`.

    A function `x` is provided a single argument:

    - clbk: callback to invoke upon function completion

    The callback function accepts any number of arguments, with the first
    argument reserved for providing an error.

    If the error argument is falsy, the function invokes a `done` callback with
    its first argument as `null` and all other provided arguments.

    If the error argument is truthy, the function invokes a function `y`. The
    number of arguments provided to `y` depends on the function's length. If `y`
    is a unary function, `y` is provided a single argument:

    - clbk: callback to invoke upon function completion

    Otherwise, `y` is provided two arguments:

    - error: the error from `x`
    - clbk: callback to invoke upon function completion

    The callback function accepts any number of arguments, with the first
    argument reserved for providing an error.

    If the error argument is falsy, the `done` callback is invoked with its
    first argument equal to `null` and all other arguments provided by `y`.

    If the error argument is truthy, the `done` callback is invoked with only
    the error argument provided by `y`.

    Execution is *not* guaranteed to be asynchronous. To guarantee asynchrony,
    wrap the `done` callback in a function which either executes at the end of
    the current stack (e.g., `nextTick`) or during a subsequent turn of the
    event loop (e.g., `setImmediate`, `setTimeout`).

    Parameters
    ----------
    x: Function
        Function to invoke.

    y: Function
        Function to invoke if `x` returns an error.

    done: Function
        Callback to invoke upon completion.

    Examples
    --------
    > function x( clbk ) {
    ...     setTimeout( onTimeout, 0 );
    ...     function onTimeout() {
    ...         clbk( new Error( 'beep' ) );
    ...     }
    ... };
    > function y( clbk ) {
    ...     setTimeout( onTimeout, 0 );
    ...     function onTimeout() {
    ...         clbk( null, 'boop' );
    ...     }
    ... };
    > function done( error, result ) {
    ...     if ( error ) {
    ...         throw error;
    ...     }
    ...     console.log( result );
    ... };
    > {{alias}}( x, y, done )
    'boop'

    See Also
    --------