restore the sharing of sequences. Shrinks the grammar by ~45%

src/runtime/c/pgf/phrasetable.h

@@ -0,0 +1,20 @@
+#ifndef PHRASETABLE_H
+#define PHRASETABLE_H
+
+class PgfSequence;
+class PgfSequenceItor;
+typedef ref<Node<PgfSequence>> PgfPhrasetable;
+
+PGF_INTERNAL_DECL
+PgfPhrasetable phrasetable_internalize(PgfPhrasetable table, ref<PgfSequence> *seq);
+
+PGF_INTERNAL_DECL
+ref<PgfSequence> phrasetable_get(PgfPhrasetable table, size_t seq_id);
+
+PGF_INTERNAL_DECL
+void phrasetable_iter(PgfPhrasetable table, PgfSequenceItor* itor, size_t *p_next_id, PgfExn *err);
+
+PGF_INTERNAL_DECL
+void phrasetable_release(PgfPhrasetable table);
+
+#endif