Added doc on %onerror, and some other minor mods

doc/LLgen/LLgen.n

@@ -2,11 +2,11 @@
 .\"	Run this paper off with
 .\"	refer [options] -p LLgen.refs LLgen.doc | [n]eqn | tbl | (nt)roff -ms
 .if '\*(>.'' \{\
-.	if '\*(.>'' \{\
+.	if '\*(<.'' \{\
 .		if n .ds >. .
 .		if n .ds >, ,
-.		if t .ds .> .
+.		if t .ds <. .
-.		if t .ds ,> ,\
+.		if t .ds <, ,\
 \}\
 \}
 .cs 5 22u
@@ -75,10 +75,10 @@ will be called
 for the rest of this document.
 It is assumed that the reader has some knowledge of LL(1) grammars and
 recursive descent parsers.
-For a survey on the subject, see .
+For a survey on the subject, see reference
-.[
+.[ (
 griffiths
-.]
+.]).
 .PP
 Extended LL(1) parsers are an extension of LL(1) parsers. They are
 derived from an Extended Context-Free (ECF) syntax instead of a Context-Free
@@ -92,7 +92,7 @@ Section 4 provides a description of the way the
 \fILLgen\fR
 user can associate
 actions with the syntax. These actions must be written in the programming
-language C ,
+language C,
 .[
 kernighan ritchie
 .]
@@ -117,15 +117,15 @@ Appendix A gives a summary of the
 input syntax.
 Appendix B gives an example.
 It is very instructive to compare this example with the one
-given in .
+given in reference
-.[
+.[ (
 yacc
-.]
+.]).
 It demonstrates the struggle \fILLparse\fR and other LL(1)
 parsers have with expressions.
 Appendix C gives an example of the \fILLgen\fR features
 allowing the user to recompile only those output files that
-have changed, using the \fImake\fR program .
+have changed, using the \fImake\fR program.
 .[
 make
 .]
@@ -396,10 +396,10 @@ To avoid name-conflicts with identifiers generated by
 Error Recovery
 .PP
 The error recovery technique used by \fILLgen\fR is a
-modification of the one presented in .
+modification of the one presented in reference
-.[
+.[ (
 automatic construction error correcting
-.]
+.]).
 It is based on \fBdefault choices\fR, which just are
 what the word says, default choices at
 every point in the grammar where there is a
@@ -551,10 +551,10 @@ The conflicts can be examined by inspecting the verbose
 The conflicts can be resolved by rewriting the grammar
 or by using \fBconflict resolvers\fR.
 The mechanism described here is based on the attributed parsing
-of .
+of reference
-.[
+.[ (
 milton
-.]
+.]).
 .PP
 An alternation conflict can be resolved by putting an \fBif condition\fR
 in front of the first conflicting production.
@@ -650,6 +650,17 @@ lexical analyzer, with the proper attributes.
 So, the lexical analyzer must have a facility to push back one
 token.
 .PP
+The user may also supply his own error recovery routines, or handle
+errors differently. For this purpose, the name of a routine to be called
+when an error occurs may be declared using the keyword \fB%onerror\fR.
+This routine takes one parameter, which is either the token number of the
+token expected, or 0. In the last case, the error occurred at a choice.
+In both cases, the routine must ensure that the next call to the lexical
+analyser returns the token that replaces the current one. Of course,
+that could well be the current one, in which case
+.I LLparse
+recovers from the error.
+.PP
 The user must supply a lexical analyzer to read the input stream and
 break it up into tokens, which are passed to
 .I LLparse.
@@ -670,7 +681,7 @@ The default name for the lexical analyzer is "yylex".
 The reason for this funny name is that a useful tool for constructing
 lexical analyzers is the
 .I Lex
-program ,
+program,
 .[
 lex
 .]
@@ -689,6 +700,8 @@ The lexical analyzer must signal the end
 of input to \fILLparse\fR
 by returning a number less than or equal to zero.
 .bp
+.SH
+References
 .[
 $LIST$
 .]
@@ -724,6 +737,7 @@ version of \fILLgen\fR is written with \fILLgen\fR.
 %token  PREFER;        /* %prefer */
 %token  DEFAULT;       /* %default */
 %token  LEXICAL;       /* %lexical */
+%token  ONERROR;       /* %onerror */
 %token  FIRST;         /* %first */
 
 /*
@@ -753,6 +767,9 @@ declaration
         | LEXICAL
                 IDENTIFIER
           ';'
+        | ONERROR
+                IDENTIFIER
+	  ';'
         | rule
         ;