Hidden and Avoidable Rectangles, BUG+1

A Hidden Rectangle is a Unique Rectangle in which one corner holds only the two rectangle digits. If one of the digits appears only inside the rectangle in the row and column of the opposite corner, the other digit can be removed from that opposite corner.

An Avoidable Rectangle uses solved cells instead of cells with two candidates. A cell you solved yourself (not a given) could be swapped, so a rectangle of two such cells with the matching digits cannot be completed to a deadly pattern. Type 1: three corners fit, and the fourth cannot take the digit that would complete the pattern.

BUG+1 (bivalue universal grave plus one): if every unsolved cell had two candidates and every digit appeared exactly twice in each unit, the puzzle would have several solutions. If all unsolved cells have two candidates except one with three, the digit that appears three times in its row, column and box has to go in that cell, so its other two candidates can be removed.

Tip: All of these only work when the puzzle has exactly one solution. The app marks such hints, and puzzles you import are checked for a unique solution first.

Notation

Hidden Rectangle: 7 can be removed from r9c3.