understanding first premise using preferential preference

[kartik vermun]

I am trying to understand the first premise in pyke. 

Assume I have various shapes of various colors-

source.source_value(circle,red)

source.source_value(circle,green)

source.source_value(circle,blue)

and I also have a preference as blue,green,red.

If there's a shape in blue I'll keep it, else will search for green else finally red.

I have the following rule:

select_shape_colors

    foreach

        source.source_value($shape,$color)

    $color_preference = get_color_preference()

    forall $color in color_preference

        first

            source.source_value($shape2,$color2)

    assert

        source.final_shape_color($shape2,$color2)

fc_extras

    def get_color_preference():

        return ('blue','green','red')

however the rule fails due to syntax and logical errors. Does anyone have any idea what I'm doing wrong?

Thanks