theorem Th29:
  a * (b * L) = (a * b) * L
proof
  let v;
  thus (a * (b * L)).v = a * (b * L).v by Def9
    .= a * (b * L.v) by Def9
    .= a * b * L.v
    .= ((a * b) * L).v by Def9;
end;
