Add double-stack words (2dup, 2drop, 2swap, 2over) and forget

tests/forth/definitions.rs

@@ -1,4 +1,5 @@
 use super::harness::*;
+use cagire::forth::Value;
 
 #[test]
 fn define_and_use_word() {
@@ -113,3 +114,44 @@ fn define_word_with_conditional() {
     f.evaluate("10 maybe-double", &ctx).unwrap();
     assert_eq!(stack_int(&f), 20);
 }
+
+#[test]
+fn forget_removes_word() {
+    let f = forth();
+    let ctx = default_ctx();
+    f.evaluate(": double 2 * ;", &ctx).unwrap();
+    f.evaluate("5 double", &ctx).unwrap();
+    assert_eq!(stack_int(&f), 10);
+    f.clear_stack();
+    f.evaluate("\"double\" forget", &ctx).unwrap();
+    f.evaluate("double", &ctx).unwrap();
+    let stack = f.stack();
+    assert_eq!(stack.len(), 1);
+    match &stack[0] {
+        Value::Str(s, _) => assert_eq!(s, "double"),
+        other => panic!("expected Str, got {:?}", other),
+    }
+}
+
+#[test]
+fn forget_nonexistent_is_noop() {
+    let f = forth();
+    let ctx = default_ctx();
+    f.evaluate("\"nosuchword\" forget", &ctx).unwrap();
+    f.evaluate("42", &ctx).unwrap();
+    assert_eq!(stack_int(&f), 42);
+}
+
+#[test]
+fn forget_and_redefine() {
+    let f = forth();
+    let ctx = default_ctx();
+    f.evaluate(": foo 10 ;", &ctx).unwrap();
+    f.evaluate("foo", &ctx).unwrap();
+    assert_eq!(stack_int(&f), 10);
+    f.clear_stack();
+    f.evaluate("\"foo\" forget", &ctx).unwrap();
+    f.evaluate(": foo 20 ;", &ctx).unwrap();
+    f.evaluate("foo", &ctx).unwrap();
+    assert_eq!(stack_int(&f), 20);
+}