diff --git a/src/bin/bitart/expression.rs b/src/bin/bitart/expression.rs
index 1cc7740..64085ce 100644
--- a/src/bin/bitart/expression.rs
+++ b/src/bin/bitart/expression.rs
@@ -23,7 +23,7 @@ impl fmt::Display for Literal {
     }
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Variable {
     X,
     Y,
@@ -93,6 +93,9 @@ impl UnaryOperation {
     fn eval(&self, env: &Env) -> i64 {
         self.operator.apply(self.operand.eval(env))
     }
+    fn contains_var(&self, variable: Variable) -> bool {
+        self.operand.contains_var(variable)
+    }
 }
 impl fmt::Display for UnaryOperation {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -180,6 +183,9 @@ impl BinaryOperation {
         self.operator
             .apply(self.operands[0].eval(env), self.operands[1].eval(env))
     }
+    fn contains_var(&self, variable: Variable) -> bool {
+        self.operands.iter().any(|e| e.contains_var(variable))
+    }
 }
 impl fmt::Display for BinaryOperation {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -207,6 +213,14 @@ impl InnerExpression {
             InnerExpression::BinaryOperation(op) => op.eval(env),
         }
     }
+    pub fn contains_var(&self, variable: Variable) -> bool {
+        match self {
+            InnerExpression::Literal(_) => false,
+            InnerExpression::Variable(v) => *v == variable,
+            InnerExpression::UnaryOperation(op) => op.contains_var(variable),
+            InnerExpression::BinaryOperation(op) => op.contains_var(variable),
+        }
+    }
 }
 impl fmt::Display for InnerExpression {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
diff --git a/src/bin/bitart/main.rs b/src/bin/bitart/main.rs
index 31a0d82..4a40c0e 100644
--- a/src/bin/bitart/main.rs
+++ b/src/bin/bitart/main.rs
@@ -18,6 +18,7 @@ use pixelfoo_apps::color::Color;
 mod expression;
 use expression::Env;
 use expression::RandomExpressionBuilder;
+use expression::Variable;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 enum Square {
@@ -129,7 +130,10 @@ fn main() -> std::io::Result<()> {
 
                 let percent = 100 * max_count / bbox.area();
 
-                if (min_percent..=max_percent).contains(&percent) {
+                if expression.contains_var(Variable::X)
+                    && expression.contains_var(Variable::Y)
+                    && (min_percent..=max_percent).contains(&percent)
+                {
                     eprintln!("chose expression {expression}");
 
                     old_board = new_board;