[DGD] error reporting with parse_string()

[Carter Cheng]

Hi Noah,

Thanks for this. It does give some insight into how parse_string works- though I am still uncertain how to generate error messages 
in cases where the file is yet unparseable. The only technique I know to cope with this is some form of local error correction and
dumping the state of the parser to some extent where the correction takes place. My knowledge of coping with errors in
parsers is limited and books I have on hand seem to suggest that most parsers rely on special procedures to cope with it. Anyhow
thanks again.

Regards,

Carter Cheng.