no-op is available via the tcllib control structure module.

AMG: I'm not sure if this is how tcllib's no-op is implemented, but it can be done very simply:

 proc no-op {args} {}

I believe the bytecode compiler specially recognizes and optimizes this.

GraemeP: That is how tcllib does it. There is a discussion on interp alias about creating a no-op, and a command that just returns its args.