(I made this word up, but you should use it.)
A: What are you doing on Thursday?
B: (Thinking: I can't come up with anything!) I don't know.
A: Let's go see a movie
B: That would be great. (I'm trapped. I hope he doesn't call, but I'll tell him a family member is ailing if he presses it.)
The two-way, generic plancel:
A: It was so good to see you.
B: You too. Let's do dinner sometime. (We're never doing dinner).
A: Definitely. (We're never doing dinner.)