In the below code, I can understand the meaning of (<), but cannot
understand the meaning of (';~:;<). Why line 3 keeps 4 rows, while line
4 keeps 6 rows? What is the meaning of "~:" here? t:([] c1:`a`b`c`d`e`f;
c2:1 1 1 2 2 2; c3:10 20 30 40 50
60);...