BUG Type 1
This deduction assumes the puzzle has exactly one solution and that all earlier candidate removals were sound. In BUG Type 1, one trivalue cell contains a single guardian candidate whose hypothetical removal would leave the full BUG structure. The guardian must be true, so the other two candidates in that cell can be removed.
What is BUG Type 1?
The premise is more than “all cells bivalue except one.” After hypothetically removing the guardian candidate, every unsolved cell must be bivalue and each remaining digit must occur exactly twice in every row, column, and box where it occurs. That is the BUG structure excluded by uniqueness.
A single trivalue cell is the usual Type 1 exception. One of its three candidates is the guardian: removing that one hypothetically produces the verified BUG structure. The guardian must stay and becomes the cell’s value; the other two, non-guardian candidates are removed.
How to spot it
Find the lone trivalue cell, then test each possible guardian by removing it hypothetically and checking the entire BUG occurrence structure. When exactly one removal produces the BUG state, that removed candidate is the guardian. Keep it and remove the other two candidates from the cell.
The logic
If the guardian were false, the remaining candidate state would be a BUG and would not support a unique completion. Under the stated uniqueness assumption, the guardian must therefore be true. The candidates removed from the escape cell are the two non-guardian candidates, not the guardian itself.