(defvar database
      '((b1 shape brick)
        (b1 color green)
        (b1 size small)
        (b1 supported-by b2)
        (b1 supported-by b3)
        (b2 shape brick)
        (b2 color red)
        (b2 size small)
        (b2 supports b1)
        (b2 left-of b3)
        (b3 shape brick)
        (b3 color red)
        (b3 size small)
        (b3 supports b1)
        (b3 right-of b2)
        (b4 shape pyramid)
        (b4 color blue)
        (b4 size large)
        (b4 supported-by b5)
        (b5 shape cube)
        (b5 color green)
        (b5 size large)
        (b5 supports b4)
        (b6 shape brick)
        (b6 color purple)
        (b6 size large)))