Your error looks like there might be an extra space after the line continuation (backslash) in the example.
The trace command is split in to 2 lines for readability but the web page's formatting introduced a hidden space if you try to copy/paste the example in to a tcl script.
The trace command is split in to 2 lines for readability but the web page's formatting introduced a hidden space if you try to copy/paste the example in to a tcl script.