def crop(clip, x1=None, y1=None, x2=None, y2=None, width=None, height=None,
    x_center=None, y_center=None):
    """
    Returns a new clip in which just a rectangular subregion of the
    original clip is conserved. x1,y1 indicates the top left corner and
    x2,y2 is the lower right corner of the croped region.
    All coordinates are in pixels. Float numbers are accepted.
    
    To crop an arbitrary rectangle:
    
    >>> crop(clip, x1=50, y1=60, x2=460, y2=275)
    
    Only remove the part above y=30:
    
    >>> crop(clip, y1=30)
    
    Crop a rectangle that starts 10 pixels left and is 200px wide
    
    >>> crop(clip, x1=10, width=200)
    
    Crop a rectangle centered in x,y=(300,400), width=50, height=150 :
    
    >>> crop(clip,  x_center=300 , y_center=400,
                        width=50, height=150)
    
    Any combination of the above should work, like for this rectangle
    centered in x=300, with explicit y-boundaries:
    
    >>> crop(x_center=300, width=400, y1=100, y2=600)
    
    """
    if width and (x1 is not None or x2 is not None):
        if x1 is not None:
            x2 = x1 + width
        else:
            x1 = x2 - width
    if height and (y1 is not None or y2 is not None):
        if y1 is not None:
            y2 = y1 + height
        else:
            y1 = y2 - height
    if x_center:
        x1, x2 = x_center - width / 2, x_center + width / 2
    if y_center:
        y1, y2 = y_center - height / 2, y_center + height / 2
    if x1 is None:
        x1 = 0
    if y1 is None:
        y1 = 0
    if x2 is None:
        x2 = clip.size[0]
    if y2 is None:
        y2 = clip.size[1]
    return clip.fl_image(lambda pic: pic[int(y1):int(y2), int(x1):int(x2)],
        apply_to=['mask'])
