Lean: issue429 test fails

[protoben]

The issue429 Lean test fails with the following type error:

error: ././././Out.lean:102:31: application type mismatch
  get_value32 x
argument
  x
has type
  two_bitvectors k_n : Type
but is expected to have type
  two_bitvectors 32 : Type

[Alasdair]

For reference, is a regression test for this issue: #429