The body of the first clause is then set up as a query, and the trace continues. From this point the trace proceeds exactly as it did for the compound query in the previous chapter. This causes the listener to look for other clauses whose heads match the query pattern. Again, although traces usually don't tell us so, it is building a query from the body of the second clause. To better understand the relationship, we will slowly step through the process of transferring control.
Subscripts identify the variable levels.
Both variables were set to be equal to each other. This is exactly what happens here. This might be implemented by setting both variables to a common internal variable. If either one takes on a new value, both take on a new value. This is fundamentally different from the assignment statements that set variable values in most computer languages.