Using the rex builtin 2-ary test function
//   Expression to be tested          Desired result          
test(assoc("octahedron", polyhedra), ["octahedron",   [8, 3]]);


Sample session:

turing ~:1> rex polyhedra.rex
ok: assoc("octahedron", [[cube, [6, 4]], [dodecahedron, [12, 5]],
 [icosahedron, [20, 3]], [octahedron, [8, 3]],
 [tetrahedron, [4, 3]]]) ==> [octahedron, [8, 3]]
polyhedra.rex loaded
1 rex > ^D Control-D means end-of-input
turing ~:1>

If there were an error, it would be indicated with ÒbadÓ instead
of ÒokÓ and the error count would be reported at the end.