21. The machine-readable medium of claim 17 , wherein the first command is received before the second command, and the distinguishing comprises: performing the first command upon receipt when the preview control specifies the effects of interaction are to be previewed; undoing the first command in conjunction with receipt of the second command; performing the second command after undoing the first command; and redoing the first command after performing the second command; wherein the second command precedes the first command in the command sequence.
21. The machine-readable medium of claim 17 , wherein the first command is received before the second command, and the distinguishing comprises: performing the first command upon receipt when the preview control specifies the effects of interaction are to be previewed; undoing the first command in conjunction with receipt of the second command; performing the second command after undoing the first command; and redoing the first command after performing the second command; wherein the second command precedes the first command in the command sequence. 22. The machine-readable medium of claim 21 , wherein undoing the first command comprises undoing the first command upon receipt of a pass-through control command received between the first and second commands.