x1 (set of tuples) is the x4-Winsorized data formed from data set x2 (set of tuples), Winsorizing with respect to variable x3.
x4 (li; between 0 inclusive and 1 inclusive) is the threshold; any and only x3 values which are more extreme than what x4 allows will be replaced. In other words, any x3 value in/as an entry of a tuple in x2 which is less than the ((1-x4)/2)th-quantile value of x3 in/from set x2 is replaced by the ((1-x4)/2)th-quantile value of x3 in/from set x2; any x3 value in/as an entry of a tuple in x2 which is greater than the (1-((1-x4)/2))th-quantile value of x3 in/from set x2 is reppaced by the (1-((1-x4)/2))th-quantile value of x3 in/from set x2; all other values of x3 in/as an entry of a tuple in x2 remain unchanged; the newly generated set of tuples is x1.