Fixed variable parse order

src/context.rs

@@ -13,19 +13,29 @@ impl Context {
 	}
 
 	pub fn push_hist(&mut self, t: Expression) { self.history.push(t); }
-	pub fn push_var(&mut self, s: String, t: Expression) { self.variables.insert(s, t); }
+	pub fn push_var(&mut self, s: String, t: Expression) -> Result<(), ()> {
+		if self.valid_varible(&s) {
+			self.variables.insert(s, t);
+			return Ok(());
+		} else { return Err(()); }
+	}
 	pub fn del_var(&mut self, s: &String) { self.variables.remove(s); }
 
 	pub fn get_variable(&self, s: &String) -> Option<Expression> {
-
 		let v: Option<&Expression>;
-
 		if s == "ans" {
 			v = self.history.last();
 		} else {
 			v = self.variables.get(s);
 		}
-
 		if v.is_some() { Some(v.unwrap().clone()) } else { None }
 	}
+
+	pub fn valid_varible(&self, s: &str) -> bool {
+		return match s {
+			"ans" => false,
+			_ => true
+		}
+	}
+
 }