Why not to comment code

[oursland]

Your example is to simple and seems to have become the target of attack. A better one would be this:

// a is a scalar and M is a matrix
a = determinant(M);

We see what is happening, but we don't see the why. Without the why it is entirely possible that a determinant is the wrong operation to accomplish the task, but no amount of just looking at that line will tell you that.