/// <summary>
/// Expand matrix to full storage (symmetric matrix is expected to store upper part only).
/// </summary>
public static SparseMatrix Expand(this SparseMatrix A)
{
if (A.Any((i, j, a) => i > j && a != 0.0))
{
throw new Exception("Expected matrix to be upper.");
}
// Transpose A.
var T = A.Transpose();
// Remove diagonal.
T.Keep((i, j, a) => i != j);
return((SparseMatrix)A.Add(T));
}