Fix remaining tests

Author: saulshanabrook

python/egglog/declarations.py

@@ -517,6 +517,7 @@ class ConstructorDecl:
 @dataclass(frozen=True)
 class UnboundVarDecl:
     name: str
+    egg_name: str | None = None
 
 
 @dataclass(frozen=True)

python/egglog/egraph.py

@@ -609,8 +609,8 @@ def _fn_decl(
         else resolve_literal(
             return_type,
             merge(
-                RuntimeExpr.__from_values__(decls, TypedExprDecl(return_type.to_just(), UnboundVarDecl("old"))),
-                RuntimeExpr.__from_values__(decls, TypedExprDecl(return_type.to_just(), UnboundVarDecl("new"))),
+                RuntimeExpr.__from_values__(decls, TypedExprDecl(return_type.to_just(), UnboundVarDecl("old", "old"))),
+                RuntimeExpr.__from_values__(decls, TypedExprDecl(return_type.to_just(), UnboundVarDecl("new", "new"))),
             ),
             lambda: decls,
         )

python/egglog/egraph_state.py

@@ -405,9 +405,8 @@ def _expr_to_egg(self, expr_decl: ExprDecl) -> bindings._Expr:  # noqa: PLR0912,
         match expr_decl:
             case LetRefDecl(name):
                 res = bindings.Var(span(), f"{name}")
-            case UnboundVarDecl(name):
-                # Prefix vars with an _ to avoid name conflicts
-                res = bindings.Var(span(), f"_{name}")
+            case UnboundVarDecl(name, egg_name):
+                res = bindings.Var(span(), egg_name or f"_{name}")
             case LitDecl(value):
                 l: bindings._Literal
                 match value: