It could also be that Dafny is not able to prove the code without some extra lemmas provided by you. Just because the code is correct does not mean that Dafny can automatically prove that it is correct.
If you want to look at some examples of proofs in Dafny then there are many in the "test" directory in the source code https://dafny.codeplex.com/SourceControl/latest#Test/
I also have a few articles on my blog documenting some features and approaches I found useful:
http://www.lexicalscope.com/blog/category/dafny/
If you want to look at some examples of proofs in Dafny then there are many in the "test" directory in the source code https://dafny.codeplex.com/SourceControl/latest#Test/
I also have a few articles on my blog documenting some features and approaches I found useful:
http://www.lexicalscope.com/blog/category/dafny/