[elcapitan]

As others have pointed out, probably more the intricacies of the existing business logic, plus the understanding of the mainframe systems that it usually runs on. Otherwise you could just build a cross compiler and call it day.

[Suppafly]

>As others have pointed out, probably more the intricacies of the existing business logic, plus the understanding of the mainframe systems that it usually runs on.

Honestly just knowing COBOL is how you get your foot into a well paying job where you learn that stuff from the old timers.