function to replace the command line text

[Dave Yost <Dave@xxxxxxxx>]

Is there a builtin function that replaces the command line with the function's output? The completion mechanism must use such a function, so there has to be something like that.

I want to use that function to build a function I can use to run a demo consisting of a sequence of commands.
 * Make an array of strings
 * For each step of the demo
    * Type a command or a keyboard shortcut that grabs the next string from the array and places it on the command line
    * Hit Enter to execute the command

Thanks