This method extends the idea of a Hidden Singles. Consider a **house** (row, column or square) where two
candidates are only seen in two of the cells. Then it is certain that both candidates will be
placed in these two cells, but we do not know in which order. In any case, no other candidates
can be in the two cells and we can eliminate them.

In this example consider row **G**. The cells **G5** and **G6** are the only ones
in the row which contain both **3** and **6**. Hence we know that **{3, 6}** will
be in these two cells in some order, but no other candidates can be in these cells.
Hence, we can eliminate all other candidates in these two cells.

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As with all basic methods, we can look for a Hidden Pair in rows, columns and squares. In the
example below we see a Hidden Pair consisting of candidates **{3, 8}** in the bottom-right square.
Both **3** and **8** are only seen twice in the square, namely in **G8** and **I7**.
Therefore, these two cells will surely contain both candidates and we can eliminate **1**
and **7** from **I7**.

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