We describe a program for the display and exploration of complex, domainspecific information: ytracc, an interactive grammar debugging tool for compiler writers. The ytracc system provides the designer of a yacc grammar a method of tracing a parser as it uses the grammar, ytracc captures the states of the parse as it is carried out. The captured parse can then be replayed forwards or backwards, stepbystep, or subtreebysubtree, as defined by the nonterminals of the grammar. The tool has been successfully used by students as an assistant in an advanced undergraduate compiler construction class, and we use the tool in our everyday work. Copyright 1991 John Wiley & Sons, Ltd
