Many programmers have had to deal with an overwritten variable resulting for example from an aliasing problem. The culprit is obviously the last write-access to that memory location before the manifestation of the bug. The usual technique for removing such bugs starts with the debugger by (1) finding the last write and (2) moving the control point of execution back to that time by re-executing the program from the beginning. We call the feature to implement this process "reverse watchpoint" and believe automating it is important. In this paper we propose a new concept, "position", that is, a point in the program execution trace, as needed for step (2) above. The position enables debuggers to automate the control of program execution to support common debugging activities. We have implemented position in Java with a bytecode transformer. Measurements show that position can be provided with an acceptable amount of overhead.