Early in my software career, I was debugging a compiler problem. Version X worked, version X+1 did not. I debugged by tracing both step by step, in two gdb sess

Early in my software career, I was debugging a compiler problem. Version X worked, version X+1 did not. I debugged by tracing both step by step, in two gdb sessions, looking for when behavior diverged. Pure torture, and that was an extremely simple case -- the processes were not communicating in any way. All the usual problems, but doubled. Oops, I stepped too far, start over. Oops, the other one stepped too far, start over. Oops, start over, oops, start over. Never again.