Given the following sets with names
s1={x1,x2}
x1={y1,y2}
y1 ={z1,z2}
When s1 is entered, the names would be replaced finally by all the sets. That is Input s1
output::{{{z1,z2},y2},x2} Question1: could we control a replacement step in any step? For example, we get {{y1,y2},x2} so that y1 is not replaced? Question2:: could we get the number of the replacement step, for example,to know, at step 2, we get {{y1,y2},x2}?