The problem with the old implementation was this:
diff = (coeff_ - other.coeff_) is a difference between pointers of type Field *.
Therefore, this difference includes an integer division (by sizeof( Field )).
If the pointers were not aligned on sizeof( Field ) boundary, rounding occurs.
Next thing other.rows_[ i ] + diff results in remultiplying with this size, bout
the roundoff error persists and might, hence, lead to a segmentation fault.

The solution was easy: use (other.rows_[ i ] - other.coeff_) for the difference.
Since other.rows_[ i ] = k * sizeof( Field ) + other.coeff_ for some k, this division
is safe.

[[Imported from SVN: r873]]