SeText generator crashes on shortcut that uses itself
The following SeText specification reproduces the issue:
```
@scanner test.Scanner;
@shortcut x = "a{x}b";
@terminals:
@eof;
A = "c{x}d";
end
```
Issues:
* It crashes the text editor.
* It crashes the generator.
* They produce stack overflow exceptions.
Note that a shortcut can only use other shortcuts defined *before* it, not *after* it.
issue