If a condition save file for tank loading has been written using the WRITE (LOADS) command, but one of its tanks has later been removed from the vessel geometry, then a variety of fatal error messages would be triggered when running the save file, such as:

"No such part" - trapped by the PART command for missing part (error# 911). "No part name" - trapped by LOAD, CONT, TYPE etc. if no part (error# 912); "Bad part name" - trapped by LOAD, CONT, TYPE etc. for bad part (error# 913).

Each of these three error conditions may be recognized by its unique error number, which is assigned to the system variable ERROR after error detection. If an ESC macro has been defined before running the condition save file, this macro will be executed automatically following an error, which permits special handling of these specific ERROR numbers, such as ignoring them or reporting them as non-fatal warnings.

Here is a sample ESC macro which ignores all missing-part errors in loading condition save files, while treating other errors as fatal:

SET ERROR=-3 MACRO ESC
IF {ERROR}>=911 THEN IF {ERROR}<=912 THEN PART OFF | SET ERROR=-3 | EXIT
ERROR
/

Let's look at this macro in more detail to understand why it works. Setting the ERROR variable to -3 causes execution to continue with the next line after any error, without reporting it or waiting. But before continuing, the ERROR variable is set equal to any special number for the error (or zero if none), and any ESC macro is executed.

While running the condition save file, if a PART command is encountered for a missing part, then the ERROR variable is set to 911 and the ESC macro is executed. Since 911 is between 911 and 912, the ESC macro first turns off the current part selection, which is very important so that upcoming LOAD, CONT, etc. commands won't affect the wrong part. Then the ERROR variable is set back to -3, and the ESC macro exits to continue save file execution.

The condition save file will normally contain a series of LOAD, CONT, etc. commands following the missing PART command. Since no part is selected (due to PART OFF in the ESC macro), each of these commands triggers an error which sets the ERROR variable to 912. The ESC macro treats 912 errors the same as 911 errors, turning PART OFF, resetting ERROR to -3, and continuing.

If any other error is encountered in the condition save file, the ERROR variable value would not be 911 or 912, so the IF line would be bypassed and the ERROR command would be reached. The ERROR command reports and reprocesses the last error detected - and since the ERROR variable no longer has a negative value, the fatal error ends condition save file execution.

Here is another sample ESC macro which reports missing-part errors with non-fatal warnings, but ignores "No part name" errors in subsequent LOAD, CONT, TYPE, etc. commands following each missing-part error:

SET ERROR=-3 MACRO ESC
VARIABLE SKIPERR
IF {ERROR}>=911 THEN IF {ERROR}<=912 THEN SET SKIPERR=1
IF {ERROR}=911 THEN SET ERROR=-2 | ERROR
IF {SKIPERR}=1 THEN PART OFF | SET ERROR=-3 | EXIT
ERROR
/

This ESC macro is similar to the previous version except a missing-part error (ERROR number 911) receives special handling, setting the ERROR variable to -2 and reprocessing the error. Setting ERROR to -2 causes "No such part" to be reported and to wait before continuing. (Note that ERROR set to -1 could have been used instead to continue without waiting.) After reporting the missing-part error command, the current part is turned off, the ERROR variable is reset to -3, and condition save file execution resumes.

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