RepeatedNull appears to be buggy (versions 8.0, and 9.0). Define f as follows: ...
This expression (explained here by Leonid) evaluates fine unless Evaluation->Debugger is enabled: h[x : (_Integer) : 1] := x Syntax::sntxf: "x:" cannot be ...
Take for example head[b_[PatternSequence[_, _]]] := 9 I know this example serves no purpose as is. It also happens in useful examples such as ...