diff --git a/README.md b/README.md
index 03b263a..c5a7321 100644
--- a/README.md
+++ b/README.md
@@ -11,13 +11,16 @@ The chase algorithm is a fundamental technique in context of database theory and
 - Ontology-based data access (OBDA)
 - Datalog with existential rules
 
-This implementation provides a **restricted chase** that guarantees termination even with existential rules by tracking applied triggers.
+This implementation provides a restricted-chase style materialization with active-trigger checks.
+The default entrypoints use a `10_000` step safeguard and report incomplete results with
+`terminated == false` if that limit is reached.
 
 ### Features
 
 - **Core Data Types**: Terms, Atoms, Rules, and Instances
 - **Existential Quantification**: Automatic generation of labeled nulls
-- **Restricted Chase**: Termination guarantees via trigger tracking
+- **Restricted Chase Style**: Active-trigger checks for existential rules
+- **Configurable Step Limit**: `chase_with_config` exposes the step bound
 - **Fluent API**: `RuleBuilder` for readable rule construction
 - **Interactive Frontends**: REPL, script runner, and a local GUI
 - **Zero Dependencies**: Pure Rust with no external runtime dependencies
@@ -65,6 +68,29 @@ assert!(result.terminated);
 println!("Derived {} facts", result.instance.len());
 ```
 
+Use `chase_with_config` to change the default step bound:
+
+```rust
+use chase_rs::{Atom, ChaseConfig, Instance, Term, chase_with_config};
+use chase_rs::chase::rule::RuleBuilder;
+
+let instance: Instance = vec![Atom::new("P", vec![Term::constant("a")])]
+    .into_iter()
+    .collect();
+let rule = RuleBuilder::new()
+    .when("P", vec![Term::var("X")])
+    .then("Q", vec![Term::var("X")])
+    .build();
+
+let result = chase_with_config(
+    instance,
+    &[rule],
+    ChaseConfig { max_steps: 100 },
+);
+
+assert!(result.terminated);
+```
+
 #### Existential Rules
 
 Rules with head-only variables (existential quantification) automatically generate fresh labeled nulls:
diff --git a/src/chase/instance.rs b/src/chase/instance.rs
index e45f645..69a154d 100644
--- a/src/chase/instance.rs
+++ b/src/chase/instance.rs
@@ -1,55 +1,99 @@
 //! A database instance is a set of ground atoms (facts).
 
-use std::collections::HashSet;
+use std::collections::{HashMap, HashSet};
+use std::error::Error;
 use std::fmt;
 
 use super::atom::Atom;
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum InstanceError {
+    NonGroundFact(Atom),
+}
+
+impl fmt::Display for InstanceError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            InstanceError::NonGroundFact(atom) => {
+                write!(f, "facts must be ground atoms: {}", atom)
+            }
+        }
+    }
+}
+
+impl Error for InstanceError {}
+
 /// A database instance containing ground atoms.
 #[derive(Debug, Clone, Default)]
 pub struct Instance {
-    facts: HashSet<Atom>,
+    facts_by_predicate: HashMap<String, HashSet<Atom>>,
+    len: usize,
 }
 
 impl Instance {
     /// Create an empty instance.
     pub fn new() -> Self {
         Instance {
-            facts: HashSet::new(),
+            facts_by_predicate: HashMap::new(),
+            len: 0,
         }
     }
 
+    /// Try to add a fact to the instance. Returns true if the fact was new.
+    pub fn try_add(&mut self, fact: Atom) -> Result<bool, InstanceError> {
+        if !fact.is_ground() {
+            return Err(InstanceError::NonGroundFact(fact));
+        }
+
+        let bucket = self
+            .facts_by_predicate
+            .entry(fact.predicate.clone())
+            .or_default();
+        let inserted = bucket.insert(fact);
+        if inserted {
+            self.len += 1;
+        }
+
+        Ok(inserted)
+    }
+
     /// Add a fact to the instance. Returns true if the fact was new.
+    ///
+    /// Panics if the atom is not ground.
     pub fn add(&mut self, fact: Atom) -> bool {
-        debug_assert!(fact.is_ground(), "Facts must be ground atoms");
-        self.facts.insert(fact)
+        self.try_add(fact).expect("facts must be ground atoms")
     }
 
     /// Check if the instance contains a fact.
     pub fn contains(&self, fact: &Atom) -> bool {
-        self.facts.contains(fact)
+        self.facts_by_predicate
+            .get(&fact.predicate)
+            .is_some_and(|facts| facts.contains(fact))
     }
 
     /// Get the number of facts.
     pub fn len(&self) -> usize {
-        self.facts.len()
+        self.len
     }
 
     /// Check if the instance is empty.
     pub fn is_empty(&self) -> bool {
-        self.facts.is_empty()
+        self.len == 0
     }
 
     /// Iterate over all facts.
     pub fn iter(&self) -> impl Iterator<Item = &Atom> {
-        self.facts.iter()
+        self.facts_by_predicate
+            .values()
+            .flat_map(|facts| facts.iter())
     }
 
     /// Get all facts with a given predicate.
     pub fn facts_for_predicate(&self, predicate: &str) -> Vec<&Atom> {
-        self.facts
-            .iter()
-            .filter(|a| a.predicate == predicate)
+        self.facts_by_predicate
+            .get(predicate)
+            .into_iter()
+            .flat_map(|facts| facts.iter())
             .collect()
     }
 }
@@ -57,7 +101,7 @@ impl Instance {
 impl fmt::Display for Instance {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         writeln!(f, "Instance {{")?;
-        for fact in &self.facts {
+        for fact in self.iter() {
             writeln!(f, "  {}", fact)?;
         }
         write!(f, "}}")
@@ -116,4 +160,15 @@ mod tests {
         assert_eq!(instance.facts_for_predicate("Person").len(), 1);
         assert_eq!(instance.facts_for_predicate("Other").len(), 0);
     }
+
+    #[test]
+    fn test_try_add_rejects_non_ground_facts() {
+        let mut instance = Instance::new();
+        let fact = Atom::new("Parent", vec![Term::var("X"), Term::constant("bob")]);
+
+        let error = instance.try_add(fact).unwrap_err();
+
+        assert!(matches!(error, InstanceError::NonGroundFact(_)));
+        assert!(instance.is_empty());
+    }
 }
diff --git a/src/chase/mod.rs b/src/chase/mod.rs
index 818f017..78a44b9 100644
--- a/src/chase/mod.rs
+++ b/src/chase/mod.rs
@@ -9,8 +9,8 @@ pub mod term;
 mod engine;
 
 pub use atom::Atom;
-pub use engine::{ChaseResult, chase};
-pub use instance::Instance;
+pub use engine::{ChaseConfig, ChaseResult, chase, chase_with_config};
+pub use instance::{Instance, InstanceError};
 pub use rule::Rule;
 pub use substitution::Substitution;
 pub use term::Term;
diff --git a/src/frontend/language.rs b/src/frontend/language.rs
index e41cebd..efe76c2 100644
--- a/src/frontend/language.rs
+++ b/src/frontend/language.rs
@@ -53,6 +53,9 @@ pub fn parse_command(input: &str) -> Result<Command, String> {
 
     if let Some(rest) = strip_keyword(trimmed, "fact") {
         let atom = parse_atom(trim_suffix(rest, '.')?)?;
+        if !atom.is_ground() {
+            return Err("facts must be ground atoms".to_string());
+        }
         return Ok(Command::Fact(atom));
     }
 
@@ -346,6 +349,12 @@ mod tests {
         }
     }
 
+    #[test]
+    fn parse_fact_command_rejects_variables() {
+        let error = parse_command("fact Parent(?X, bob).").unwrap_err();
+        assert_eq!(error, "facts must be ground atoms");
+    }
+
     #[test]
     fn parse_rule_command() {
         let command = parse_command("rule P(?X), Q(?X, a) -> R(?X).").unwrap();
diff --git a/src/frontend/provenance.rs b/src/frontend/provenance.rs
index 6bb4618..42642f2 100644
--- a/src/frontend/provenance.rs
+++ b/src/frontend/provenance.rs
@@ -3,9 +3,7 @@
 use std::collections::{HashMap, HashSet};
 
 use crate::chase::substitution::unify_atom;
-use crate::{Atom, ChaseResult, Instance, Rule, Substitution, Term};
-
-const DEFAULT_MAX_STEPS: usize = 10_000;
+use crate::{Atom, ChaseConfig, ChaseResult, Instance, Rule, Substitution, Term};
 
 #[derive(Debug)]
 pub struct MaterializedState {
@@ -76,12 +74,13 @@ pub fn materialize(base_instance: Instance, rules: &[Rule]) -> MaterializedState
         .cloned()
         .map(|fact| (fact, Derivation::Input))
         .collect::<HashMap<_, _>>();
+    let max_steps = ChaseConfig::default().max_steps;
     let mut null_gen = NullGenerator::seeded_from(&instance, rules);
     let mut applied_triggers = HashSet::new();
     let mut steps = 0;
 
     loop {
-        if steps >= DEFAULT_MAX_STEPS {
+        if steps >= max_steps {
             return MaterializedState {
                 result: ChaseResult {
                     instance,
diff --git a/src/frontend/session.rs b/src/frontend/session.rs
index 0aa887d..f3b3d2b 100644
--- a/src/frontend/session.rs
+++ b/src/frontend/session.rs
@@ -41,7 +41,10 @@ impl Session {
         match command {
             Command::Fact(atom) => {
                 self.materialized = None;
-                let inserted = self.base_instance.add(atom.clone());
+                let inserted = self
+                    .base_instance
+                    .try_add(atom.clone())
+                    .map_err(|err| err.to_string())?;
                 let action = if inserted {
                     "Added"
                 } else {
diff --git a/src/lib.rs b/src/lib.rs
index 6e5d103..e2b217e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,4 +2,7 @@ pub mod chase;
 pub mod frontend;
 
 // Re-export main types for convenience
-pub use chase::{Atom, ChaseResult, Instance, Rule, Substitution, Term, chase};
+pub use chase::{
+    Atom, ChaseConfig, ChaseResult, Instance, InstanceError, Rule, Substitution, Term, chase,
+    chase_with_config,
+};