    op(502,xfx,'~|'),
    op(503,xfx,~&),
    op(504,xfx,=>),
    op(504,xfx,<=),
    op(505,xfx,<=>),
    op(505,xfx,<~>),
    op(550,xfx,:=).


weight(f(x,y)) = weight(x) + call(func, x)
