GrammaticalExpression shouldn't / can't be frozen

src/ultk/language/grammar.py

@@ -51,7 +51,7 @@ def __str__(self) -> str:
         return out_str
 
 
-@dataclass(eq=True, frozen=True, kw_only=True)
+@dataclass(eq=True, kw_only=True)
 class GrammaticalExpression(Expression):
     """A GrammaticalExpression has been built up from a Grammar by applying a sequence of Rules.
     Crucially, it is _callable_, using the functions corresponding to each rule.

src/ultk/language/language.py

@@ -18,16 +18,16 @@
 from dataclasses import dataclass
 from typing import Callable, Iterable
 
-@dataclass(frozen=True, kw_only=True)
+@dataclass()
 class Expression:
 
     """Minimally contains a form and a meaning."""
 
     # gneric/dummy form and meaning if not specified
     # useful for hashing in certain cases
     # (e.g. a GrammaticalExpression which has not yet been evaluate()'d and so does not yet have a Meaning)
-    form: str = ""
-    meaning: Meaning = Meaning(tuple(), Universe(tuple()))
+    form: str | None = ""
+    meaning: Meaning | None = Meaning(tuple(list()), Universe(tuple(list())))
 
     def can_express(self, referent: Referent) -> bool:
         """Return True if the expression can express the input single meaning point and false otherwise."""
@@ -48,7 +48,7 @@ def __lt__(self, other: object) -> bool:
 
     def __bool__(self) -> bool:
         return bool(self.form and self.meaning)
-
+    
 
 class Language:
     """Minimally contains Expression objects."""