The MARKUP SOURCE being processed is not well-formed.

#6143 Run-time catchable error

The MARKUP SOURCE being processed is not well-formed.

Additional Information

A MARKUP-REGION-EVENT was left unclosed. The innermost unclosed event is 'X'.
The end of a MARKUP-REGION-EVENT does not have a corresponding start. The event is 'X'.
Encountered the end of MARKUP-REGION-EVENT 'X' when the last started region is 'Y'.
The MARKUP-REGION-EVENT 'X' appears more than once.
where X and Y are descriptions of markup events.

Explanation

This error is reported if the markup source input given to do markup-parse is not well-formed. A well-formed markup stream must satisfy the following conditions:

Every #markup-end signal must be preceded by a #markup-start signal with the same (reference-equal) markup-region-event argument in the same stream, and vice versa: every #markup-start must be followed by the corresponding #markup-end before the end of the markup stream.
The #markup-start and #markup-end signals must be properly nested.
If there are two #markup-start signals using the same markup-region-event argument, there must be a #markup-end with the same argument between them. In other words, a region cannot be restarted without being ended first. The opposite rule also holds: two #markup-end signals with the same event must be separated by a #markup-start, so no region can be ended twice.