    lenT [1,2,3,4,5]
==> lenHelp ([1,2,3,4,5], 0)
==> lenHelp ([2,3,4,5], 0+1)
==> lenHelp ([2,3,4,5], 1)
==> lenHelp ([3,4,5], 1+1)
==> lenHelp ([3,4,5], 2)
==> lenHelp ([4,5], 2+1)
==> lenHelp ([4,5], 3)
==> lenHelp ([5], 3+1)
==> lenHelp ([5], 4)
==> lenHelp ([], 4+1)
==> lenHelp ([], 5)
==> 5