# Control a replacement step in any step and get the number of steps?

GROUPS:
 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 s1output::{{{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}?