implement "if: ~value:" to negate it. ("!" doesnt work for YAML reasons)

src/chats/serenity.yaml

@@ -180,11 +180,13 @@
 - Please find yourself a cozy place to drift.
 - Do you have a reservation?
 - label: reservation
-- Reservation? Is there not enough space for everybody?:
+- if: ~$reservation
+  Reservation? Is there not enough space for everybody?:
   - Ah, space there is.
   - But I can't get overworked, can I?
   - I'm running this joint all by myself, after all.
   - Apart of good ol' Clippy.
+  - set: $reservation
   - goto: reservation
 - No reservation. Can I still buy something?:
   - '"Buy"? Ah, this old earth thing.'

src/var.rs

@@ -10,6 +10,7 @@ pub const TOKEN_GREATER_THAN: &str = ">";
 pub const TOKEN_LESS_THAN: &str = "<";
 pub const TOKEN_GREATER_EQUALS: &str = ">=";
 pub const TOKEN_LESS_EQUALS: &str = "<=";
+pub const TOKEN_NEGATE: &str = "~";
 
 pub const DEFAULT_CHAT_SPEED: f32 = 10.0;
 
@@ -297,13 +298,18 @@ impl GameVars {
             // Got something like "if $somevar:".
             // Check whether the variable evaluates to true.
             let part = parts[0];
+            let (part, negate) = if part.starts_with(TOKEN_NEGATE) {
+                (&part[1..], true)
+            } else {
+                (part, false)
+            };
             if part.contains(SCOPE_SEPARATOR) {
                 let part = Self::normalize_varname(scope, part);
                 let value_bool = self.getb(part.as_str());
-                return value_bool;
+                return value_bool ^ negate;
             }
             else {
-                return Self::evaluate_str_as_bool(part);
+                return Self::evaluate_str_as_bool(part) ^ negate;
             }
 
         } else if parts.len() == 2 {