Example using assoc:
// Definition of polyhedra

polyhedra =
    [ ["cube",         [6, 4]],
      ["dodecahedron", [12, 5]],
      ["icosahedron",  [20, 3]],
      ["octahedron",   [8, 3]],
      ["tetrahedron",  [4, 3]]
   ];

// Expression to be evaluated

assoc("octahedron", polyhedra);

// Expected result:

["octahedron", [8, 3]]