New example by Jacob Frost, tidied by lcp

Now prove_goalw_cterm calls print_sign_exn_unit, so that the rest of the error message ("The exception above was raised for...") will appear. And print_exn calls print_sign_exn_unit so that it can re-raise the SAME exception.

Updated the "version" variable (which was never done for Isabelle-94 revisions 1 and 2!)

Redefined functions to suppress redundant leading 0s and 1s

new in mixfix annotations: "' " (quote space) separates delimiters without adding extra white space for printing;

Added the exception handler handle _ => exit 1. This will catch all errors and force an exit with error code, causing Make to fail.