how to implement 20 - point and point - 20 using coerce()?

In Ruby, the operation of

    point - 20
    20 - point

are to be implemented.

But the following code:



    class Point

      attr_accessor :x, :y

      def initialize(x,y)
        @x, @y = x, y
      end

      def -(q)
      if (q.is_a? Fixnum)
        return Point.new(@x - q, @y - q)
      end
        Point.new(@x - q.x, @y - q.y)
      end

      def -@
        Point.new(-@x, -@y)
      end

      def *(c)
        Point.new(@x * c, @y * c)
      end

      def coerce(something)
        [self, something]
      end

    end

    p = Point.new(100,100)
    q = Point.new(80,80)

    p (-p)

    p p - q
    p q - p

    p p * 3
    p 5 * p

    p p - 30
    p 30 - p

Output:

    #<Point:0x2424e54 @x=-100, @y=-100>
    #<Point:0x2424dc8 @x=20, @y=20>
    #<Point:0x2424d3c @x=-20, @y=-20>
    #<Point:0x2424cc4 @x=300, @y=300>
    #<Point:0x2424c38 @x=500, @y=500>
    #<Point:0x2424bc0 @x=70, @y=70>
    #<Point:0x2424b20 @x=70, @y=70>

"30 - p" will actually be taken as "p - 30" by the coerce function.  Can
it be made to work?

I am actually surprise that the "-" method won't coerce the argument
this way:

    class Fixnum
      def -(something)
        if (/* something is unknown class */)
          a, b = something.coerce(self)
          return -(a - b)   # because we are doing a - b but we wanted b
- a, so it is negated
        end
      end
    end

that is, the function returns a "negated version of a - b" instead of
just returning "a - b".
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