[SMT-LIB] concrete syntax of 'success'

[cok at frontiernet.net]

The SMTLIBv2 standard is clear in its informal text that setting the :print-success option to false suppresses printing 'success' when that would be the result of a command that would respond with 'success'.

However, the definition of <gen_response> in the concrete syntax does not allow for this. it says that the response is unsupported |  success |  ( error <string> )
I think that this definition needs to be expanded to allow an empty response, and that the mapping of the abstract syntax 'success' is to either concrete 'success' or empty.

- David