diff --git a/Cargo.toml b/Cargo.toml
index 4328d3d..80142a4 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -30,4 +30,5 @@ rand = "0.8.5"
 regex = "1.10.2"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
-serde-xml-rs = "0.6.0"
\ No newline at end of file
+serde-xml-rs = "0.6.0"
+num-rational = "0.4.2"
diff --git a/src/lib.rs b/src/lib.rs
index 363d529..0254b2f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -3,6 +3,7 @@
 pub mod bma_model;
 pub mod json_model;
 pub mod traits;
+pub mod update_fn;
 pub mod xml_model;
 
 mod _impl_bma_model;
diff --git a/src/update_fn/bma_fn_tree.rs b/src/update_fn/bma_fn_tree.rs
new file mode 100644
index 0000000..8ebeddc
--- /dev/null
+++ b/src/update_fn/bma_fn_tree.rs
@@ -0,0 +1,120 @@
+use crate::update_fn::enums::{AggregateOp, ArithOp, Literal, UnaryOp};
+use crate::update_fn::tokenizer::BmaFnToken;
+use std::cmp;
+use std::fmt;
+
+/// Enum of possible node types in a BMA expression syntax tree.
+///
+/// In particular, a node type can be:
+///     - A "terminal" node containing a literal (variable, constant).
+///     - A "unary" node with a `UnaryOp` and a sub-expression.
+///     - A binary "arithmetic" node, with a `BinaryOp` and two sub-expressions.
+///     - An "aggregation" node with a `AggregateOp` op and a list of sub-expressions.
+#[derive(Clone, Debug, Eq, Hash, PartialEq)]
+pub enum Expression {
+    Terminal(Literal),
+    Unary(UnaryOp, Box<BmaFnNode>),
+    Arithmetic(ArithOp, Box<BmaFnNode>, Box<BmaFnNode>),
+    Aggregation(AggregateOp, Vec<Box<BmaFnNode>>),
+}
+
+/// A single node in a syntax tree of a FOL formula.
+///
+/// Each node tracks its:
+///     - `height`; A positive integer starting from 0 (for term nodes).
+///     - `expression_tree`; A parse tree for the expression`.
+///     - `function_str`; A canonical string representation of the expression.
+#[derive(Clone, Debug, Eq, Hash, PartialEq)]
+pub struct BmaFnNode {
+    pub function_str: String,
+    pub height: u32,
+    pub expression_tree: Expression,
+}
+
+impl BmaFnNode {
+    /// "Parse" a new [BmaFnNode] from a list of [BmaFnToken] objects.
+    pub fn from_tokens(_tokens: &[BmaFnToken]) -> Result<BmaFnNode, String> {
+        todo!()
+    }
+
+    /// Create a "unary" [BmaFnNode] from the given arguments.
+    ///
+    /// See also [Expression::Unary].
+    pub fn mk_unary(child: BmaFnNode, op: UnaryOp) -> BmaFnNode {
+        let subform_str = format!("{op}({child})");
+        BmaFnNode {
+            function_str: subform_str,
+            height: child.height + 1,
+            expression_tree: Expression::Unary(op, Box::new(child)),
+        }
+    }
+
+    /// Create a "binary" arithmetic [BmaFnNode] from the given arguments.
+    ///
+    /// See also [Expression::Binary].
+    pub fn mk_arithmetic(left: BmaFnNode, right: BmaFnNode, op: ArithOp) -> BmaFnNode {
+        BmaFnNode {
+            function_str: format!("({left} {op} {right})"),
+            height: cmp::max(left.height, right.height) + 1,
+            expression_tree: Expression::Arithmetic(op, Box::new(left), Box::new(right)),
+        }
+    }
+
+    /// Create a [BmaFnNode] representing a Boolean constant.
+    ///
+    /// See also [Expression::Terminal] and [Atomic::True] / [Atomic::False].
+    pub fn mk_constant(constant_val: i32) -> BmaFnNode {
+        Self::mk_literal(Literal::Int(constant_val))
+    }
+
+    /// Create a [BmaFnNode] representing a variable.
+    ///
+    /// See also [Expression::Terminal] and [Literal::Str].
+    pub fn mk_variable(var_name: &str) -> BmaFnNode {
+        Self::mk_literal(Literal::Str(var_name.to_string()))
+    }
+
+    /// A helper function which creates a new [BmaFnNode] for the given [Literal] value.
+    fn mk_literal(literal: Literal) -> BmaFnNode {
+        BmaFnNode {
+            function_str: literal.to_string(),
+            height: 0,
+            expression_tree: Expression::Terminal(literal),
+        }
+    }
+
+    /// Create a [BmaFnNode] representing an aggregation operator applied to given arguments.
+    pub fn mk_aggregation(op: AggregateOp, inner_nodes: Vec<BmaFnNode>) -> BmaFnNode {
+        let max_height = inner_nodes
+            .iter()
+            .map(|node| node.height)
+            .max()
+            .unwrap_or(0);
+        let child_expressions: Vec<String> = inner_nodes
+            .iter()
+            .map(|child| child.function_str.clone())
+            .collect();
+        let args_str = child_expressions.join(", ");
+        let function_str = format!("{}({})", op, args_str);
+
+        let inner_boxed_nodes = inner_nodes.into_iter().map(Box::new).collect();
+
+        BmaFnNode {
+            function_str,
+            height: max_height + 1,
+            expression_tree: Expression::Aggregation(op, inner_boxed_nodes),
+        }
+    }
+}
+
+impl BmaFnNode {
+    pub fn as_str(&self) -> &str {
+        self.function_str.as_str()
+    }
+}
+
+impl fmt::Display for BmaFnNode {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.function_str)
+    }
+}
diff --git a/src/update_fn/enums.rs b/src/update_fn/enums.rs
new file mode 100644
index 0000000..849bcb5
--- /dev/null
+++ b/src/update_fn/enums.rs
@@ -0,0 +1,70 @@
+use serde::{Deserialize, Serialize};
+use std::fmt;
+
+#[derive(Clone, Debug, Eq, Hash, PartialEq, Serialize, Deserialize)]
+pub enum Literal {
+    Int(i32),
+    Str(String),
+}
+
+impl fmt::Display for Literal {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Literal::Int(value) => write!(f, "{}", value),
+            Literal::Str(value) => write!(f, "{}", value),
+        }
+    }
+}
+
+#[derive(Clone, Debug, Eq, Hash, PartialEq, Serialize, Deserialize)]
+pub enum ArithOp {
+    Add,
+    Minus,
+    Times,
+    Div,
+}
+
+impl fmt::Display for ArithOp {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            ArithOp::Add => write!(f, "+"),
+            ArithOp::Minus => write!(f, "-"),
+            ArithOp::Times => write!(f, "*"),
+            ArithOp::Div => write!(f, "/"),
+        }
+    }
+}
+
+#[derive(Clone, Debug, Eq, Hash, PartialEq, Serialize, Deserialize)]
+pub enum UnaryOp {
+    Ceil,
+    Floor,
+    Abs,
+}
+
+impl fmt::Display for UnaryOp {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            UnaryOp::Ceil => write!(f, "ceil"),
+            UnaryOp::Floor => write!(f, "floor"),
+            UnaryOp::Abs => write!(f, "abs"),
+        }
+    }
+}
+
+#[derive(Clone, Debug, Eq, Hash, PartialEq, Serialize, Deserialize)]
+pub enum AggregateOp {
+    Min,
+    Max,
+    Avg,
+}
+
+impl fmt::Display for AggregateOp {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            AggregateOp::Min => write!(f, "min"),
+            AggregateOp::Max => write!(f, "max"),
+            AggregateOp::Avg => write!(f, "avg"),
+        }
+    }
+}
diff --git a/src/update_fn/mod.rs b/src/update_fn/mod.rs
new file mode 100644
index 0000000..185e2ea
--- /dev/null
+++ b/src/update_fn/mod.rs
@@ -0,0 +1,4 @@
+pub mod bma_fn_tree;
+mod enums;
+mod parser;
+mod tokenizer;
diff --git a/src/update_fn/parser.rs b/src/update_fn/parser.rs
new file mode 100644
index 0000000..65b3dba
--- /dev/null
+++ b/src/update_fn/parser.rs
@@ -0,0 +1 @@
+// todo
diff --git a/src/update_fn/tokenizer.rs b/src/update_fn/tokenizer.rs
new file mode 100644
index 0000000..6ca52b5
--- /dev/null
+++ b/src/update_fn/tokenizer.rs
@@ -0,0 +1,13 @@
+use crate::update_fn::enums::{AggregateOp, ArithOp, Literal, UnaryOp};
+
+/// Enum of all possible tokens occurring in a BMA function string.
+#[derive(Clone, Debug, Eq, Hash, PartialEq)]
+pub enum BmaFnToken {
+    Atomic(Literal),
+    Unary(UnaryOp),
+    Binary(ArithOp),
+    Aggregate(AggregateOp, Vec<BmaFnToken>),
+    TokenList(Vec<BmaFnToken>),
+}
+
+// todo