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Segment information for a pseudoresidue, exposed when a segment is added, and then deleted while the window is displayed.

A pseudoresidue is a residue or segment that Foldit uses for internal purposes.

Normally, pseudoresidues aren't seen in the user interface, and can't be detected by a recipe.

In some cases, signs of a pseudoresidue may be seen when segments are added or deleted.

The image shown here was captured by Formula350, who describes the procedure for Puzzle 1930:

I can only access that residue's information if I add one residue to the Linker puzzle, any more and it just displays a blank info window.
The trick is to add the one residue, then open Seg Info on the last residue on the puzzle -- which in this case is 241 -- and then Reset Puzzle.
What was 241, now becomes 240, and what was 242, now becomes 241, thereby popping up in the viewer.
(From Foldit Discord #bugs-and-feedback, 20 December 2020)

Pseudoresidues also seem to occur in symmetry puzzles. In a symmetry puzzle, the segment numbers returned by band.GetResidueEnd and structure.GetHBonds may return segment (residue) numbers greater than the value returned by structure.GetCount, indicating the segment is on a symmetric chain. The segment numbers are not continuous, skipping one segment between the main chain and each symmetric chain. These "spacer" segments are likely another example of pseudoresidues. Messages seen in the file log.txt for a symmetry puzzle also seem to refer to these "missing" segments.