nonExist now does the expected thing

src/runtime/c/pgf/data.h

@@ -180,7 +180,8 @@ typedef enum {
 	PGF_SYMBOL_LIT,
 	PGF_SYMBOL_VAR,
 	PGF_SYMBOL_KS,
-	PGF_SYMBOL_KP
+	PGF_SYMBOL_KP,
+	PGF_SYMBOL_NE
 } PgfSymbolTag;
 
 typedef struct {
@@ -208,6 +209,8 @@ typedef struct PgfSymbolKP
 	 * symbol. */
 } PgfSymbolKP;
 
+typedef struct {
+} PgfSymbolNE;
 
 typedef GuSeq PgfSequence; // -> PgfSymbol
 typedef GuList(PgfSequence) PgfSequences;

src/runtime/c/pgf/linearizer.c

@@ -532,6 +532,10 @@ pgf_lzr_linearize(PgfConcr* concr, PgfCncTree ctree, size_t lin_idx, PgfLinFuncs
 				}
 				break;
 			}
+			case PGF_SYMBOL_NE: {
+				// Nothing to be done here
+				break;
+			}
 			default:
 				gu_impossible(); 
 			}

src/runtime/c/pgf/parser.c

@@ -163,6 +163,8 @@ pgf_prev_extern_sym(PgfSymbol sym)
 		return *((PgfSymbol*) (((PgfSymbolLit*) i.data)+1));
 	case PGF_SYMBOL_VAR:
 		return *((PgfSymbol*) (((PgfSymbolVar*) i.data)+1));
+	case PGF_SYMBOL_NE:
+		return *((PgfSymbol*) (((PgfSymbolNE*) i.data)+1));
 	default:
 		gu_impossible();
 		return gu_null_variant;
@@ -1410,6 +1412,10 @@ pgf_parsing_symbol(PgfParseState* before, PgfParseState* after,
 	case PGF_SYMBOL_VAR:
 		// XXX TODO proper support
 		break;
+	case PGF_SYMBOL_NE: {
+		// Nothing to be done here
+		break;
+	}
 	default:
 		gu_impossible();
 	}
@@ -2743,6 +2749,7 @@ pgf_parser_leftcorner_item(PgfLeftcornerFn* clo, PgfItem* item)
 				break;
 			}
 			case PGF_SYMBOL_LIT:
+			case PGF_SYMBOL_NE:
 				// Nothing to be done here
 				break;
 			case PGF_SYMBOL_VAR:

src/runtime/c/pgf/printer.c

@@ -258,6 +258,10 @@ pgf_print_symbol(PgfSymbol sym, GuWriter *wtr, GuExn *err)
 		gu_printf(wtr, err, "<%d,$%d>", svar->d, svar->r);
 		break;
 	}
+	case PGF_SYMBOL_NE: {
+		gu_puts("nonExist", wtr, err);
+		break;
+	}
 	default:
 		gu_impossible();
 	}

src/runtime/c/pgf/reader.c

@@ -717,6 +717,13 @@ pgf_read_symbol(PgfReader* rdr)
 		}
 		break;
 	}
+	case PGF_SYMBOL_NE: {
+		gu_new_variant(PGF_SYMBOL_NE,
+		               PgfSymbolNE,
+		               &sym, rdr->opool);
+		gu_return_on_exn(rdr->err, gu_null_variant);
+		break;
+	}
 	default:
 		pgf_read_tag_error(rdr);
 	}