The database needs to be parsed with incremental mode for grammar par…

…sing

metamath-rs/src/database.rs

@@ -160,6 +160,8 @@ pub struct DbOptions {
     /// use it.  If this is false, any reparse will result in a full
     /// recalculation, so it is always safe but different settings will be
     /// faster for different tasks.
+    ///
+    /// Note that incremental mode is required for grammar parsing.
     pub incremental: bool,
     /// Number of jobs to run in parallel at any given time.
     pub jobs: usize,
@@ -712,6 +714,10 @@ impl Database {
 
     /// Builds and returns the grammar.
     pub fn grammar_pass(&mut self) -> &Arc<Grammar> {
+        assert!(
+            self.options.incremental,
+            "The database needs to be parsed with incremental mode for grammar parsing."
+        );
         if self.grammar.is_none() {
             self.name_pass();
             self.scope_pass();