diff --git a/src/lib.rs b/src/lib.rs
index 5763096..be08590 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -6,6 +6,7 @@ pub mod test_utils; // TODO:
                     //   to have it accessible from outside binaries.
 mod expression_components;
 mod symbolic_domains;
+mod system;
 mod xml_parsing;
 
 #[cfg(test)]
diff --git a/src/system.rs b/src/system.rs
new file mode 100644
index 0000000..87d2983
--- /dev/null
+++ b/src/system.rs
@@ -0,0 +1,26 @@
+pub mod variable_update_function {
+    use crate::expression_components::expression::Expression;
+
+    pub struct VariableUpdateFn<T> {
+        // pub input_vars_names: Vec<String>, // todo what for? remove by default if not needed; uncomment if needed
+        // pub target_var_name: String, // todo this should not care about this; the caller should keep this information
+        pub terms: Vec<(T, Expression<T>)>,
+        pub default: T,
+    }
+
+    impl<T> VariableUpdateFn<T> {
+        pub fn new(
+            // input_vars_names: Vec<String>,
+            // target_var_name: String,
+            terms: Vec<(T, Expression<T>)>,
+            default: T,
+        ) -> Self {
+            Self {
+                // input_vars_names,
+                // target_var_name,
+                terms,
+                default,
+            }
+        }
+    }
+}
diff --git a/src/xml_parsing/mod.rs b/src/xml_parsing/mod.rs
index 52ab2ca..6abb43b 100644
--- a/src/xml_parsing/mod.rs
+++ b/src/xml_parsing/mod.rs
@@ -1,3 +1,4 @@
 pub mod expression_parser;
 pub mod utils;
+pub mod variable_update_fn_parser;
 pub mod xml_reader;
diff --git a/src/xml_parsing/utils.rs b/src/xml_parsing/utils.rs
index 5e9e068..fa1c042 100644
--- a/src/xml_parsing/utils.rs
+++ b/src/xml_parsing/utils.rs
@@ -25,6 +25,7 @@ pub enum XmlReadingError {
     },
     UnderlyingReaderError(#[from] xml::reader::Error),
     ParsingError(String),
+    NoSuchAttribute(String),
 }
 
 impl Display for XmlReadingError {
@@ -39,6 +40,7 @@ impl Display for XmlReadingError {
                 write!(f, "Underlying reader error: {}", e)
             }
             XmlReadingError::ParsingError(s) => write!(f, "Parsing error; could not parse {}", s),
+            XmlReadingError::NoSuchAttribute(s) => write!(f, "No such attribute: {}", s),
         }
     }
 }
diff --git a/src/xml_parsing/variable_update_fn_parser.rs b/src/xml_parsing/variable_update_fn_parser.rs
new file mode 100644
index 0000000..e419713
--- /dev/null
+++ b/src/xml_parsing/variable_update_fn_parser.rs
@@ -0,0 +1,185 @@
+use std::{io::BufRead, str::FromStr};
+
+use xml::reader::XmlEvent;
+
+use crate::{
+    expression_components::expression::Expression,
+    system::variable_update_function::VariableUpdateFn,
+};
+
+use super::{
+    utils::expect_opening,
+    utils::{expect_closure_of, expect_opening_of, map_list, StartElementWrapper, XmlReadingError},
+    xml_reader::XmlReader,
+};
+
+/// expects the xml reader to be at the start of the <transition> element
+impl<T: FromStr> VariableUpdateFn<T> {
+    pub fn try_from_xml<XR: XmlReader<BR>, BR: BufRead>(
+        xml: &mut XR,
+    ) -> Result<Self, XmlReadingError> {
+        let some_start_element = expect_opening(xml)?;
+        if !matches!(
+            some_start_element.name.local_name.as_str(),
+            "listOfInputs" | "listOfOutputs"
+        ) {
+            return Err(XmlReadingError::UnexpectedEvent {
+                expected: super::utils::ExpectedXmlEvent::Start(
+                    "listOfInputs or listOfOutputs".to_string(),
+                ),
+                got: XmlEvent::StartElement {
+                    name: some_start_element.name,
+                    attributes: some_start_element.attributes,
+                    namespace: some_start_element.namespace,
+                },
+            });
+        }
+
+        // listOfInputs might not be present at all
+        let input_vars_names = if some_start_element.name.local_name == "listOfInputs" {
+            let aux = map_list(xml, "listOfInputs", "input", process_input_var_name_item)?;
+            expect_closure_of(xml, "listOfOutputs")?; // must be followed by listOfOutputs
+            aux
+        } else {
+            Vec::new()
+        };
+
+        // listOfOutputs must be present
+        let target_vars_names =
+            map_list(xml, "listOfOutputs", "output", process_output_var_name_item)?;
+        let mut target_vars_names = target_vars_names.iter();
+        let head = target_vars_names // todo head must be used; caller has no way of knowing which variable is the target otherwise
+            .next()
+            .ok_or(XmlReadingError::UnexpectedEvent {
+                expected: super::utils::ExpectedXmlEvent::Start("target var name".to_string()),
+                got: XmlEvent::EndElement {
+                    name: some_start_element.name,
+                },
+            });
+        target_vars_names.next().map_or_else(
+            || Ok::<(), ()>(()),
+            |_elem| {
+                // Err(XmlReadingError::UnexpectedEvent {
+                //     expected: super::utils::ExpectedXmlEvent::End("list of vars names".to_string()),
+                //     got: // ...,
+                // })
+                panic!("expected only one target var but found multiple; todo might want to change this")
+            },
+        );
+        // .ok_or(XmlReadingError::UnexpectedEvent { expected: super::utils::ExpectedXmlEvent::End("listOfOutputs".to_string()), got: () })
+
+        expect_opening_of(xml, "listOfFunctionTerms")?;
+        let (default, terms) = get_default_and_list_of_terms(xml)?;
+
+        expect_closure_of(xml, "transition")?;
+        // Ok(Self::new(input_vars_names, head.into(), terms, default))
+        // Ok(VariableUpdateFn::new(terms, default))
+
+        let xd = VariableUpdateFn::new(terms, default);
+
+        Ok(xd)
+    }
+}
+
+fn process_input_var_name_item<XR: XmlReader<BR>, BR: BufRead>(
+    xml: &mut XR,
+    current: StartElementWrapper,
+) -> Result<String, XmlReadingError> {
+    let mut qualitative_species = current.attributes.iter().filter_map(|att| {
+        if att.name.local_name == "qualitativeSpecies" {
+            Some(att.value.clone())
+        } else {
+            None
+        }
+    });
+
+    let item = qualitative_species
+        .next()
+        // .ok_or("expected \"qualitativeSpecies\" arg in input, but none found")?;
+        .ok_or(XmlReadingError::NoSuchAttribute(
+            "qualitativeSpecies".to_string(),
+        ))?; // todo
+
+    expect_closure_of(xml, "input")?;
+
+    Ok(item)
+}
+
+fn process_output_var_name_item<XR: XmlReader<BR>, BR: BufRead>(
+    xml: &mut XR,
+    current: StartElementWrapper,
+) -> Result<String, XmlReadingError> {
+    let mut qualitative_species = current.attributes.iter().filter_map(|att| {
+        if att.name.local_name == "qualitativeSpecies" {
+            Some(att.value.clone())
+        } else {
+            None
+        }
+    });
+
+    let item = qualitative_species
+        .next()
+        .ok_or(XmlReadingError::NoSuchAttribute(
+            "value after qualitativeSpecies".to_string(),
+        ))?;
+
+    expect_closure_of(xml, "output")?;
+
+    Ok(item)
+}
+
+type Out<T> = (T, Vec<(T, Expression<T>)>);
+
+fn get_default_and_list_of_terms<T: FromStr, XR: XmlReader<BR>, BR: BufRead>(
+    xml: &mut XR,
+) -> Result<Out<T>, XmlReadingError> {
+    // firs should be the default
+    let default_element = expect_opening_of(xml, "defaultTerm")?;
+
+    let default_val = result_level_from_attributes(&default_element)?;
+
+    expect_closure_of(xml, "defaultTerm")?;
+
+    // expect_opening_of("functionTerms", xml)?; // already inside "functionTerms" List; first item was default element
+    let values_and_expressions = map_list(
+        xml,
+        "listOfFunctionTerms",
+        "functionTerm",
+        process_function_term_item,
+    )?;
+
+    Ok((default_val, values_and_expressions))
+}
+
+fn process_function_term_item<T: FromStr, XR: XmlReader<BR>, BR: BufRead>(
+    xml: &mut XR,
+    current: StartElementWrapper,
+) -> Result<(T, Expression<T>), XmlReadingError> {
+    let res_lvl = result_level_from_attributes(&current)?;
+
+    expect_opening_of(xml, "math")?;
+    // try_from_xml expects to have the first apply already opened
+    expect_opening_of(xml, "apply")?;
+
+    let exp = Expression::try_from_xml(xml)?;
+
+    expect_closure_of(xml, "math")?;
+    expect_closure_of(xml, "functionTerm")?;
+
+    Ok((res_lvl, exp))
+}
+
+fn result_level_from_attributes<T: FromStr>(
+    elem: &StartElementWrapper,
+) -> Result<T, XmlReadingError> {
+    let attribute_with_result_lvl = elem
+        .attributes
+        .iter()
+        .find(|attr_name| attr_name.name.local_name == "resultLevel")
+        .ok_or(XmlReadingError::NoSuchAttribute("resultLevel".to_string()))?;
+
+    attribute_with_result_lvl
+        .value
+        .parse::<T>()
+        .map_err(|_| XmlReadingError::ParsingError(attribute_with_result_lvl.value))
+}