diff --git a/examples/list.ds b/examples/list.ds
index c27f974..9fbfb88 100644
--- a/examples/list.ds
+++ b/examples/list.ds
@@ -4,4 +4,4 @@ x = numbers.0
 y = numbers.1
 z = numbers.2
 
-assert_equal <x + y, z>
+(assert_equal x + y, z)
diff --git a/src/abstract_tree/function_call.rs b/src/abstract_tree/function_call.rs
index 9d13c15..f4acf2d 100644
--- a/src/abstract_tree/function_call.rs
+++ b/src/abstract_tree/function_call.rs
@@ -30,6 +30,8 @@ impl AbstractTree for FunctionCall {
             "tool" => {
                 let tool_node = name_node.child(0).unwrap();
                 let tool = match tool_node.kind() {
+                    "assert" => Tool::Assert,
+                    "assert_equal" => Tool::AssertEqual,
                     "output" => Tool::Output,
                     "read" => Tool::Read,
                     _ => panic!(""),
@@ -42,14 +44,14 @@ impl AbstractTree for FunctionCall {
 
         let mut arguments = Vec::new();
 
-        let mut current_index = 2;
-        while current_index < node.child_count() - 1 {
-            let expression_node = node.child(current_index).unwrap();
-            let expression = Expression::from_syntax_node(source, expression_node)?;
+        for index in 2..node.child_count() - 1 {
+            let child = node.child(index).unwrap();
 
-            arguments.push(expression);
+            if child.is_named() {
+                let expression = Expression::from_syntax_node(source, child)?;
 
-            current_index += 1;
+                arguments.push(expression);
+            }
         }
 
         Ok(FunctionCall { name, arguments })
@@ -59,13 +61,15 @@ impl AbstractTree for FunctionCall {
         let identifier = match &self.name {
             FunctionName::Identifier(identifier) => identifier,
             FunctionName::Tool(tool) => {
-                let value = self
-                    .arguments
-                    .first()
-                    .map(|expression| expression.run(source, context))
-                    .unwrap_or(Ok(Value::Empty))?;
+                let mut values = Vec::with_capacity(self.arguments.len());
 
-                return tool.run(&value);
+                for expression in &self.arguments {
+                    let value = expression.run(source, context)?;
+
+                    values.push(value);
+                }
+
+                return tool.run(&values);
             }
         };
         let key = identifier.inner();
diff --git a/src/abstract_tree/tool.rs b/src/abstract_tree/tool.rs
index 55df2b1..e7d8ea3 100644
--- a/src/abstract_tree/tool.rs
+++ b/src/abstract_tree/tool.rs
@@ -2,24 +2,78 @@ use std::fs::read_to_string;
 
 use serde::{Deserialize, Serialize};
 
-use crate::{Result, Value};
+use crate::{Error, Result, Value};
 
 #[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
 pub enum Tool {
+    Assert,
+    AssertEqual,
     Output,
     Read,
 }
 
 impl Tool {
-    pub fn run(&self, value: &Value) -> Result<Value> {
+    pub fn run(&self, values: &[Value]) -> Result<Value> {
         let value = match self {
+            Tool::Assert => {
+                if values.len() != 1 {
+                    return Err(Error::ExpectedToolArgumentAmount {
+                        tool_name: "assert",
+                        expected: 1,
+                        actual: values.len(),
+                    });
+                }
+
+                if values[0].as_boolean()? {
+                    Value::Empty
+                } else {
+                    return Err(Error::AssertEqualFailed {
+                        expected: Value::Boolean(true),
+                        actual: values[0].clone(),
+                    });
+                }
+            }
+            Tool::AssertEqual => {
+                if values.len() != 2 {
+                    return Err(Error::ExpectedToolArgumentAmount {
+                        tool_name: "assert_equal",
+                        expected: 2,
+                        actual: values.len(),
+                    });
+                }
+
+                if values[0] == values[1] {
+                    Value::Empty
+                } else {
+                    return Err(Error::AssertEqualFailed {
+                        expected: values[0].clone(),
+                        actual: values[1].clone(),
+                    });
+                }
+            }
             Tool::Output => {
-                println!("{value}");
+                if values.len() != 1 {
+                    return Err(Error::ExpectedToolArgumentAmount {
+                        tool_name: "output",
+                        expected: 1,
+                        actual: values.len(),
+                    });
+                }
+
+                println!("{}", values[0]);
 
                 Value::Empty
             }
             Tool::Read => {
-                let file_contents = read_to_string(value.as_string()?)?;
+                if values.len() != 1 {
+                    return Err(Error::ExpectedToolArgumentAmount {
+                        tool_name: "read",
+                        expected: 1,
+                        actual: values.len(),
+                    });
+                }
+
+                let file_contents = read_to_string(values[0].as_string()?)?;
 
                 Value::String(file_contents)
             }
diff --git a/src/error.rs b/src/error.rs
index 45cad4c..61d1694 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -47,8 +47,8 @@ pub enum Error {
     },
 
     /// A function was called with the wrong amount of arguments.
-    ExpectedFunctionArgumentAmount {
-        identifier: String,
+    ExpectedToolArgumentAmount {
+        tool_name: &'static str,
         expected: usize,
         actual: usize,
     },
@@ -283,38 +283,6 @@ impl From<toml::de::Error> for Error {
 }
 
 impl Error {
-    pub(crate) fn _expect_function_argument_amount(
-        identifier: &str,
-        actual: usize,
-        expected: usize,
-    ) -> Result<()> {
-        if actual == expected {
-            Ok(())
-        } else {
-            Err(Error::ExpectedFunctionArgumentAmount {
-                identifier: identifier.to_string(),
-                expected,
-                actual,
-            })
-        }
-    }
-
-    pub(crate) fn _expected_minimum_function_argument_amount(
-        identifier: &str,
-        actual: usize,
-        minimum: usize,
-    ) -> Result<()> {
-        if actual >= minimum {
-            Ok(())
-        } else {
-            Err(Error::ExpectedAtLeastFunctionArgumentAmount {
-                identifier: identifier.to_string(),
-                minimum,
-                actual,
-            })
-        }
-    }
-
     pub fn type_error(actual: Value, expected: &'static [ValueType]) -> Self {
         Error::TypeError { actual, expected }
     }
@@ -414,13 +382,13 @@ impl fmt::Display for Error {
                 "An operator expected {} arguments, but got {}.",
                 expected, actual
             ),
-            ExpectedFunctionArgumentAmount {
+            ExpectedToolArgumentAmount {
+                tool_name,
                 expected,
                 actual,
-                identifier,
             } => write!(
                 f,
-                "{identifier} expected {expected} arguments, but got {actual}.",
+                "{tool_name} expected {expected} arguments, but got {actual}.",
             ),
             ExpectedAtLeastFunctionArgumentAmount {
                 minimum,
diff --git a/tree-sitter-dust b/tree-sitter-dust
index a207087..c6ec1ff 160000
--- a/tree-sitter-dust
+++ b/tree-sitter-dust
@@ -1 +1 @@
-Subproject commit a207087ccafb71764b007db88580b35d5e37dae3
+Subproject commit c6ec1ff6ea51ce28ee465f9c230e2580fab4c437