In case of trouble

Unlike many programs compiled from C or FORTRAN code, the $\cal OYST\!ER\,$ code, being written in an interpreted language, almost never crashes by returning to the operating system prompt. What usually happens is that execution stops at the offending line in the script and an error message is printed. At this point, the fearless user can print values of variables, enter IDL commands, all within the current context, in order to investigate the causes of the crash. To resume the $\cal OYST\!ER\,$session by backing out of the procedure, it is best to return right away to the main level by typing retall.