diff --git a/README.md b/README.md
index 5dd5cbe..6669e13 100644
--- a/README.md
+++ b/README.md
@@ -45,7 +45,7 @@ use evalexpr::error::expect_number;
 let mut configuration = HashMapConfiguration::new();
 configuration.insert_variable("five", 5);
 configuration.insert_variable("twelve", 12);
-configuration.insert_function("f", Function::new(1 /* argument amount */, Box::new(|arguments| {
+configuration.insert_function("f", Function::new(Some(1) /* argument amount */, Box::new(|arguments| {
     if let Value::Int(int) = arguments[0] {
         Ok(Value::Int(int / 2))
     } else if let Value::Float(float) = arguments[0] {
@@ -54,7 +54,7 @@ configuration.insert_function("f", Function::new(1 /* argument amount */, Box::n
         Err(Error::expected_number(arguments[0].clone()))
     }
 })));
-configuration.insert_function("avg", Function::new(2 /* argument amount */, Box::new(|arguments| {
+configuration.insert_function("avg", Function::new(Some(2) /* argument amount */, Box::new(|arguments| {
     expect_number(&arguments[0])?;
     expect_number(&arguments[1])?;
 
@@ -115,6 +115,18 @@ Supported unary operators:
 | - | 110 | Negation |
 | ! | 110 | Logical not |
 
+#### The Aggregation Operator
+
+The aggregation operator aggregates two values into a tuple.
+If one of the values is a tuple already, the resulting tuple will be flattened.
+Example:
+
+```rust
+use evalexpr::*;
+
+assert_eq!(eval("1, 2, 3"), Ok(Value::from(vec![Value::from(1), Value::from(2), Value::from(3)])));
+```
+
 ### Values
 
 Operators take values as arguments and produce values as results.
@@ -157,7 +169,7 @@ This crate also allows to define arbitrary functions to be used in parsed expres
 A function is defined as a `Function` instance.
 It contains two properties, the `argument_amount` and the `function`.
 The `function` is a boxed `Fn(&[Value]) -> Result<Value, Error>`.
-The `argument_amount` determines the length of the slice that is passed to `function`.
+The `argument_amount` determines the length of the slice that is passed to `function` if it is `Some(_)`, otherwise the function is defined to take an arbitrary amount of arguments.
 It is verified on execution by the crate and does not need to be verified by the `function`.
 
 Be aware that functions need to verify the types of values that are passed to them.
diff --git a/src/function/mod.rs b/src/function/mod.rs
index ee282e9..d449d0b 100644
--- a/src/function/mod.rs
+++ b/src/function/mod.rs
@@ -2,13 +2,13 @@ use error::{self, Error};
 use value::Value;
 
 pub struct Function {
-    argument_amount: usize,
+    argument_amount: Option<usize>,
     function: Box<Fn(&[Value]) -> Result<Value, Error>>,
 }
 
 impl Function {
     pub fn new(
-        argument_amount: usize,
+        argument_amount: Option<usize>,
         function: Box<Fn(&[Value]) -> Result<Value, Error>>,
     ) -> Self {
         Self {
@@ -18,7 +18,10 @@ impl Function {
     }
 
     pub fn call(&self, arguments: &[Value]) -> Result<Value, Error> {
-        error::expect_function_argument_amount(arguments.len(), self.argument_amount)?;
+        if let Some(argument_amount) = self.argument_amount {
+            error::expect_function_argument_amount(arguments.len(), argument_amount)?;
+        }
+
         (self.function)(arguments)
     }
 }
diff --git a/src/lib.rs b/src/lib.rs
index a908064..0dc7c0b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -34,7 +34,7 @@
 //! let mut configuration = HashMapConfiguration::new();
 //! configuration.insert_variable("five", 5);
 //! configuration.insert_variable("twelve", 12);
-//! configuration.insert_function("f", Function::new(1 /* argument amount */, Box::new(|arguments| {
+//! configuration.insert_function("f", Function::new(Some(1) /* argument amount */, Box::new(|arguments| {
 //!     if let Value::Int(int) = arguments[0] {
 //!         Ok(Value::Int(int / 2))
 //!     } else if let Value::Float(float) = arguments[0] {
@@ -43,7 +43,7 @@
 //!         Err(Error::expected_number(arguments[0].clone()))
 //!     }
 //! })));
-//! configuration.insert_function("avg", Function::new(2 /* argument amount */, Box::new(|arguments| {
+//! configuration.insert_function("avg", Function::new(Some(2) /* argument amount */, Box::new(|arguments| {
 //!     expect_number(&arguments[0])?;
 //!     expect_number(&arguments[1])?;
 //!
@@ -158,7 +158,7 @@
 //! A function is defined as a `Function` instance.
 //! It contains two properties, the `argument_amount` and the `function`.
 //! The `function` is a boxed `Fn(&[Value]) -> Result<Value, Error>`.
-//! The `argument_amount` determines the length of the slice that is passed to `function`.
+//! The `argument_amount` determines the length of the slice that is passed to `function` if it is `Some(_)`, otherwise the function is defined to take an arbitrary amount of arguments.
 //! It is verified on execution by the crate and does not need to be verified by the `function`.
 //!
 //! Be aware that functions need to verify the types of values that are passed to them.
@@ -365,7 +365,7 @@ mod test {
         configuration.insert_function(
             "sub2".to_string(),
             Function::new(
-                1,
+                Some(1),
                 Box::new(|arguments| {
                     if let Value::Int(int) = arguments[0] {
                         Ok(Value::Int(int - 2))
@@ -407,7 +407,7 @@ mod test {
         configuration.insert_function(
             "sub2",
             Function::new(
-                1,
+                Some(1),
                 Box::new(|arguments| {
                     if let Value::Int(int) = arguments[0] {
                         Ok(Value::Int(int - 2))
@@ -422,7 +422,7 @@ mod test {
         configuration.insert_function(
             "avg",
             Function::new(
-                2,
+                Some(2),
                 Box::new(|arguments| {
                     expect_number(&arguments[0])?;
                     expect_number(&arguments[1])?;
@@ -440,7 +440,7 @@ mod test {
         configuration.insert_function(
             "muladd",
             Function::new(
-                3,
+                Some(3),
                 Box::new(|arguments| {
                     expect_number(&arguments[0])?;
                     expect_number(&arguments[1])?;