Instance[B1, Cube]
Color[B1, Red]
Instance[B2, Brick]
Color[B2, Blue]
Instance[B3, Sphere]
Color[B3, Green]
Isa[Cube, Brick]
Isa[Brick, Block]
Isa[Sphere, Block]
On[B2, Table]
On[B1, B2]
On[B3, B1]
Instance[a, Block] IF Isa[z, Block] AND Instance[a, z]
Above[x, y] IF On[x, y]
Above[x, y] IF On[x, z] AND Above[z, y]