See if a candidate is single.
single = sudoku_iscandsingle ( C , ix , jx )
a matrix of candidates
the row index of a particular candidate
the column index of a particular candidate
true if the cell C(ix,jx) is unique in its row, column and block.
This algorithm is useful when we want to insert a particular candidate in a sudoku. If the candidate is unique in its row, column or subblock, the insertion is fine. But if the sudoku is inconsistent, it may happen that the candidate appears several times in the matrix of candidates. If we did not check, we could insert it several times.