Well, I said "can have exactly the same expressive power", just meaning that a GUI is not more limited than a language just because it's a GUI. Perhaps misleadingly stated :-). Specific languages and specific GUIs naturally will vary in what they can express.
I was thinking mainly of buttons on a window that open instances of a window corresponding to the non-terminals on the RHS of a CFG production rule. That's the "nesting" (nesting can also represent iteration, or you can have explicit iteration in the form of lists). Extra constraints can be imposed on this CFG just as they are in a language.
Example of simple arithmetic expressions (a DSL): each window has a text field, and four buttons marked with the operators +,-,* and /. You can type in a number, or click a button which opens another window with the same format and labeled with the chosen operator.
I was thinking mainly of buttons on a window that open instances of a window corresponding to the non-terminals on the RHS of a CFG production rule. That's the "nesting" (nesting can also represent iteration, or you can have explicit iteration in the form of lists). Extra constraints can be imposed on this CFG just as they are in a language.
Example of simple arithmetic expressions (a DSL): each window has a text field, and four buttons marked with the operators +,-,* and /. You can type in a number, or click a button which opens another window with the same format and labeled with the chosen operator.