Ugh, I'm not looking forwards to debugging this issue:

```pycon
>>> space.parse("proc { |a, (x, y)| }").block.stmts[0].expr.block_arg.block_args
[<topaz.ast.Argument object at 0x108e569d0>, <topaz.ast.Argument object at 0x108e56ed0>]
>>> space.parse("proc { |(x, y)| }").block.stmts[0].expr.block_arg.block_args
[]
```