When the debugger is on, this predicate causes a SPYTERM-port to be displayed. In the subsequent execution, any variable modification in Var will be shown as a MODIFY-port. The SPYTERM and MODIFY-port have the same unique invocation number, therefore the invocation-skip command (i) can be used to follow the chain of modifications.
This is equivalent to
    spy_term(Var, Var->(constrained of suspend))
    
    This feature is implemented using high-priority (1) delayed goals
    which create the MODIFY-ports.  These goals are visible to the
    user as monitor_term/4 goals among the delayed goals.
[eclipse 1]: lib(fd).
yes.
[eclipse 1]: trace.
Debugger switched on - creep mode
[eclipse 3]: [X, Y] :: 1..9, spy_var(X), X #> Y, Y #> X.
  (1) 1 CALL  [X, Y] :: 1..9   %> creep
  (1) 1 EXIT  [X{[1..9]}, Y{[1..9]}] :: 1..9   %> creep
  (3) 2 SPYTERM  X{[1..9]}   %> jump to invoc: [3]? 
  (3) 3 MODIFY  X{[2..9]}   %> jump to invoc: [3]? 
  (3) 4 MODIFY  X{[4..7]}   %> jump to invoc: [3]? 
no (more) solution.