I understand everything up to the point which I've marked in red... Can anyone explain those few lines? I would greatly appreciate it! Thanks.
Seems like KCL (Kirchoff's Current Law) at node A. i1 = i0 + i2 (sum of currents entering) = (sum of currents leaving)
Oh, I see it now. I totally misunderstood the fact that they were simply substituting i0 = i1 - i2 into the third mesh equation. Thanks :-)