Fix some typing errors.

Author: hzhangxyz

ddss/egraph.py

@@ -100,8 +100,8 @@ def _execute_expr(self, data: Rule) -> typing.Iterator[Rule]:
         query = data.conclusion
         for target in self.pairs:
             if unification := target @ query:
-                result = target.ground(unification, scope="1")
-                yield _build_term_to_rule(result)
+                if result := target.ground(unification, scope="1"):
+                    yield _build_term_to_rule(result)
 
     def _execute_fact(self, data: Rule) -> typing.Iterator[Rule]:
         if len(data) != 0:
@@ -116,5 +116,7 @@ def _execute_fact(self, data: Rule) -> typing.Iterator[Rule]:
             query = _build_lhs_rhs_to_term(idea, fact)
             for target in self.pairs:
                 if unification := target @ query:
-                    result = target.ground(unification, scope="1")
-                    yield _build_term_to_rule(result.term[2])
+                    if result := target.ground(unification, scope="1"):
+                        term = result.term
+                        if isinstance(term, Term):
+                            yield _build_term_to_rule(term[2])

ddss/utility.py

@@ -1,4 +1,4 @@
-def str_rule_get_str_idea(data: str) -> str:
+def str_rule_get_str_idea(data: str) -> str | None:
     if not data.startswith("--"):
         return f"----\n{data.splitlines()[0]}\n"
     return None