Problem: if one creates two callouts at two different steps, and if one changes the placement of the first one,
then the second one is also moved. Conversely, if one moves the second one, then the first one is also moved.
Reason: after the end of the first callout, the reference to the line containing its placement is not cleared.
If there is no such line for the second callout, then the first one is still used.
Solution: clear the reference to the placement of a callout by the end of this callout.
File bug2.mpd is an example showing the bug
File bug2_patch.txt is a patch correcting the bug