diff --git a/Cargo.toml b/Cargo.toml
index 5d24aab..452d518 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,6 +13,7 @@ serde_json = "1.0.89"
 log = "0.4.17"
 env_logger="0.10.0"
 lazy_static = "1.4.0"
+rand = "0.8.5"
 
 [dev-dependencies]
 serde_yaml = "0.9.16"
diff --git a/src/builtins/comparison.rs b/src/builtins/comparison.rs
index 50c72c3..574f57e 100644
--- a/src/builtins/comparison.rs
+++ b/src/builtins/comparison.rs
@@ -41,11 +41,6 @@ use anyhow::Result;
 /// * `v1` - The first value.
 /// * `v2` - The second value.
 pub fn compare(op: &BoolOp, v1: &Value, v2: &Value) -> Result<Value> {
-    // Handle undefined values.
-    if v1 == &Value::Undefined || v2 == &Value::Undefined {
-        return Ok(Value::Undefined);
-    }
-
     // Rely on generated comparison operators.
     // The variants of Value enum are specified in the order necessary to
     // obtain the desired semantics.
diff --git a/src/builtins/mod.rs b/src/builtins/mod.rs
index d8089ad..1ce5844 100644
--- a/src/builtins/mod.rs
+++ b/src/builtins/mod.rs
@@ -4,6 +4,7 @@
 pub mod comparison;
 pub mod numbers;
 pub mod sets;
+pub mod utils;
 
 use crate::ast::Expr;
 use crate::lexer::Span;
@@ -26,8 +27,20 @@ lazy_static! {
 	m.insert("ceil", numbers::ceil);
 	m.insert("floor", numbers::floor);
 	m.insert("numbers.range", numbers::range);
-	m.insert("round", numbers::round);	
+	m.insert("rand.intn", numbers::intn);
+	m.insert("round", numbers::round);		
 
+	// sets
+	m.insert("intersection", sets::intersection_of_set_of_sets);
+	m.insert("union", sets::union_of_set_of_sets);
+	
 	m
     };
 }
+
+pub fn must_cache(path: &str) -> Option<&'static str> {
+    match path {
+        "rand.intn" => Some("rand.intn"),
+        _ => None,
+    }
+}
diff --git a/src/builtins/numbers.rs b/src/builtins/numbers.rs
index 85c3e69..be6424b 100644
--- a/src/builtins/numbers.rs
+++ b/src/builtins/numbers.rs
@@ -2,43 +2,12 @@
 // Licensed under the MIT License.
 
 use crate::ast::{ArithOp, Expr};
+use crate::builtins::utils::{ensure_args_count, ensure_numeric, ensure_string};
 use crate::lexer::Span;
 use crate::value::{Float, Value};
 
-use anyhow::{bail, Result};
-
-fn ensure_args_count(
-    span: &Span,
-    fcn: &'static str,
-    params: &[Expr],
-    args: &[Value],
-    expected: usize,
-) -> Result<()> {
-    if args.len() != expected {
-        let span = match args.len() > expected {
-            false => span,
-            true => params[args.len() - 1].span(),
-        };
-        if expected == 1 {
-            bail!(span.error(format!("`{fcn}` expects 1 argument").as_str()))
-        } else {
-            bail!(span.error(format!("`{fcn}` expects {expected} arguments").as_str()))
-        }
-    }
-    Ok(())
-}
-
-fn ensure_numeric(fcn: &str, arg: &Expr, v: &Value) -> Result<Float> {
-    Ok(match &v {
-        Value::Number(n) => n.0 .0,
-        _ => {
-            let span = arg.span();
-            bail!(
-                span.error(format!("`{fcn}` expects numeric argument. Got `{v}` instead").as_str())
-            )
-        }
-    })
-}
+use anyhow::Result;
+use rand::{thread_rng, Rng};
 
 pub fn abs(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
     ensure_args_count(span, "abs", params, args, 1)?;
@@ -92,6 +61,25 @@ pub fn round(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
     ))
 }
 
+pub fn intn(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let fcn = "rand.intn";
+    ensure_args_count(span, fcn, params, args, 2)?;
+    let _ = ensure_string(fcn, &params[0], &args[0])?;
+    let n = ensure_numeric(fcn, &params[0], &args[1])?;
+    if n != n.floor() || n < 0 as Float {
+        return Ok(Value::Undefined);
+    }
+
+    if n == 0.0 {
+        return Ok(Value::from_float(0 as Float));
+    }
+
+    // TODO: bounds checking; arbitrary precision
+    let mut rng = thread_rng();
+    let v = rng.gen_range(0..n as u64);
+    Ok(Value::from_float(v as f64))
+}
+
 pub fn arithmetic_operation(
     op: &ArithOp,
     expr1: &Expr,
@@ -99,10 +87,6 @@ pub fn arithmetic_operation(
     v1: Value,
     v2: Value,
 ) -> Result<Value> {
-    if v1 == Value::Undefined || v2 == Value::Undefined {
-        return Ok(Value::Undefined);
-    }
-
     let op_name = format!("{:?}", op).to_lowercase();
     let v1 = ensure_numeric(op_name.as_str(), expr1, &v1)?;
     let v2 = ensure_numeric(op_name.as_str(), expr2, &v2)?;
@@ -114,6 +98,8 @@ pub fn arithmetic_operation(
         ArithOp::Div if v2 == 0.0 => return Ok(Value::Undefined),
         ArithOp::Div => v1 / v2,
         ArithOp::Mod if v2 == 0.0 => return Ok(Value::Undefined),
+        ArithOp::Mod if v1.floor() != v1 => return Ok(Value::Undefined),
+        ArithOp::Mod if v2.floor() != v2 => return Ok(Value::Undefined),
         ArithOp::Mod => v1 % v2,
     }))
 }
diff --git a/src/builtins/sets.rs b/src/builtins/sets.rs
index dc2a5e3..d843b9e 100644
--- a/src/builtins/sets.rs
+++ b/src/builtins/sets.rs
@@ -2,25 +2,76 @@
 // Licensed under the MIT License.
 
 use crate::ast::Expr;
+use crate::builtins::utils::{ensure_args_count, ensure_set};
+use crate::lexer::Span;
 use crate::value::Value;
 
 use std::collections::BTreeSet;
-use std::rc::Rc;
 
 use anyhow::{bail, Result};
 
-fn ensure_set(fcn: &str, arg: &Expr, v: Value) -> Result<Rc<BTreeSet<Value>>> {
-    Ok(match v {
-        Value::Set(s) => s,
-        _ => {
-            let span = arg.span();
-            bail!(span.error(format!("`{fcn}` expects set argument. Got `{v}` instead").as_str()))
-        }
-    })
-}
-
 pub fn difference(expr1: &Expr, expr2: &Expr, v1: Value, v2: Value) -> Result<Value> {
     let s1 = ensure_set("difference", expr1, v1)?;
     let s2 = ensure_set("difference", expr2, v2)?;
     Ok(Value::from_set(s1.difference(&s2).cloned().collect()))
 }
+
+pub fn intersection(expr1: &Expr, expr2: &Expr, v1: Value, v2: Value) -> Result<Value> {
+    let s1 = ensure_set("intersection", expr1, v1)?;
+    let s2 = ensure_set("intersection", expr2, v2)?;
+    Ok(Value::from_set(s1.intersection(&s2).cloned().collect()))
+}
+
+pub fn union(expr1: &Expr, expr2: &Expr, v1: Value, v2: Value) -> Result<Value> {
+    let s1 = ensure_set("union", expr1, v1)?;
+    let s2 = ensure_set("union", expr2, v2)?;
+    Ok(Value::from_set(s1.union(&s2).cloned().collect()))
+}
+
+pub fn intersection_of_set_of_sets(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let name = "intersection";
+    ensure_args_count(span, name, params, args, 1)?;
+    let set = ensure_set(name, &params[0], args[0].clone())?;
+
+    let mut res = BTreeSet::new();
+    let mut first = true;
+
+    for s in set.iter() {
+        let s = match s {
+            Value::Set(s) => s,
+            _ => bail!(
+                span.error(format!("`{name}` expects set of sets. Got `{}`", args[0]).as_str())
+            ),
+        };
+
+        if first {
+            res = (**s).clone();
+            first = false;
+        } else {
+            res = res.intersection(s).cloned().collect();
+        }
+    }
+
+    Ok(Value::from_set(res))
+}
+
+pub fn union_of_set_of_sets(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let name = "union";
+    ensure_args_count(span, name, params, args, 1)?;
+    let set = ensure_set(name, &params[0], args[0].clone())?;
+
+    let mut res = BTreeSet::new();
+
+    for s in set.iter() {
+        let s = match s {
+            Value::Set(s) => s,
+            _ => bail!(
+                span.error(format!("`{name}` expects set of sets. Got `{}`", args[0]).as_str())
+            ),
+        };
+
+        res = res.union(s).cloned().collect();
+    }
+
+    Ok(Value::from_set(res))
+}
diff --git a/src/builtins/strings.rs b/src/builtins/strings.rs
new file mode 100644
index 0000000..f881320
--- /dev/null
+++ b/src/builtins/strings.rs
@@ -0,0 +1,19 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+use crate::value::Value;
+
+use anyhow::{bail, Result};
+
+fn ensure_numeric(fcn: &str, arg: &Expr, v: &Value) -> Result<Float> {
+    Ok(match &v {
+        Value::Number(n) => n.0 .0,
+        _ => {
+            let span = arg.span();
+            bail!(
+                span.error(format!("`{fcn}` expects numeric argument. Got `{v}` instead").as_str())
+            )
+        }
+    })
+}
+
diff --git a/src/builtins/utils.rs b/src/builtins/utils.rs
new file mode 100644
index 0000000..b5f30d3
--- /dev/null
+++ b/src/builtins/utils.rs
@@ -0,0 +1,64 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+use crate::ast::Expr;
+use crate::lexer::Span;
+use crate::value::{Float, Value};
+
+use std::collections::BTreeSet;
+use std::rc::Rc;
+
+use anyhow::{bail, Result};
+
+pub fn ensure_args_count(
+    span: &Span,
+    fcn: &'static str,
+    params: &[Expr],
+    args: &[Value],
+    expected: usize,
+) -> Result<()> {
+    if args.len() != expected {
+        let span = match args.len() > expected {
+            false => span,
+            true => params[args.len() - 1].span(),
+        };
+        if expected == 1 {
+            bail!(span.error(format!("`{fcn}` expects 1 argument").as_str()))
+        } else {
+            bail!(span.error(format!("`{fcn}` expects {expected} arguments").as_str()))
+        }
+    }
+    Ok(())
+}
+
+pub fn ensure_numeric(fcn: &str, arg: &Expr, v: &Value) -> Result<Float> {
+    Ok(match &v {
+        Value::Number(n) => n.0 .0,
+        _ => {
+            let span = arg.span();
+            bail!(
+                span.error(format!("`{fcn}` expects numeric argument. Got `{v}` instead").as_str())
+            )
+        }
+    })
+}
+
+pub fn ensure_string(fcn: &str, arg: &Expr, v: &Value) -> Result<String> {
+    Ok(match &v {
+        Value::String(s) => s.clone(),
+        _ => {
+            let span = arg.span();
+            bail!(span.error(format!("`{fcn}` expects string argument. Got `{v}` instead").as_str()))
+        }
+    })
+}
+
+pub fn ensure_set(fcn: &str, arg: &Expr, v: Value) -> Result<Rc<BTreeSet<Value>>> {
+    Ok(match v {
+        Value::Set(s) => s,
+        _ => {
+            let span = arg.span();
+            bail!(span.error(format!("`{fcn}` expects set argument. Got `{v}` instead").as_str()))
+        }
+    })
+}
diff --git a/src/interpreter.rs b/src/interpreter.rs
index 73d6459..664b773 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -29,7 +29,7 @@ pub struct Interpreter<'source> {
     default_rules: HashMap<String, Vec<(&'source Rule<'source>, Option<String>)>>,
     processed: BTreeSet<&'source Rule<'source>>,
     active_rules: Vec<&'source Rule<'source>>,
-    intns: BTreeMap<Vec<Value>, Value>,
+    builtins_cache: BTreeMap<(&'static str, Vec<Value>), Value>,
 }
 
 #[derive(Debug, Clone)]
@@ -62,7 +62,7 @@ impl<'source> Interpreter<'source> {
             default_rules: HashMap::new(),
             processed: BTreeSet::new(),
             active_rules: vec![],
-            intns: BTreeMap::new(),
+            builtins_cache: BTreeMap::new(),
         })
     }
 
@@ -264,6 +264,11 @@ impl<'source> Interpreter<'source> {
     ) -> Result<Value> {
         let lhs = self.eval_expr(lhs_expr)?;
         let rhs = self.eval_expr(rhs_expr)?;
+
+        if lhs == Value::Undefined || rhs == Value::Undefined {
+            return Ok(Value::Undefined);
+        }
+
         builtins::comparison::compare(op, &lhs, &rhs)
     }
 
@@ -273,30 +278,17 @@ impl<'source> Interpreter<'source> {
         lhs: &'source Expr<'source>,
         rhs: &'source Expr<'source>,
     ) -> Result<Value> {
-        let lhs = self.eval_expr(lhs)?;
-        let rhs = self.eval_expr(rhs)?;
+        let lhs_value = self.eval_expr(lhs)?;
+        let rhs_value = self.eval_expr(rhs)?;
 
-        let lhs = if let Value::Set(set) = lhs {
-            set
-        } else {
-            return Err(anyhow!("expect {:?} to be a set", lhs));
-        };
+        if lhs_value == Value::Undefined || rhs_value == Value::Undefined {
+            return Ok(Value::Undefined);
+        }
 
-        let rhs = if let Value::Set(set) = rhs {
-            set
-        } else {
-            return Err(anyhow!("expect {:?} to be a set", rhs));
-        };
-
-        info!(
-            "eval_bin_expr, op: {:?}, lhs: {:?}, rhs: {:?}",
-            op, lhs, rhs
-        );
-
-        Ok(Value::from_set(match op {
-            BinOp::Or => lhs.union(&rhs).cloned().collect(),
-            BinOp::And => lhs.intersection(&rhs).cloned().collect(),
-        }))
+        match op {
+            BinOp::Or => builtins::sets::union(lhs, rhs, lhs_value, rhs_value),
+            BinOp::And => builtins::sets::intersection(lhs, rhs, lhs_value, rhs_value),
+        }
     }
 
     fn eval_arith_expr(
@@ -308,6 +300,10 @@ impl<'source> Interpreter<'source> {
         let lhs_value = self.eval_expr(lhs)?;
         let rhs_value = self.eval_expr(rhs)?;
 
+        if lhs_value == Value::Undefined || rhs_value == Value::Undefined {
+            return Ok(Value::Undefined);
+        }
+
         match (op, &lhs_value, &rhs_value) {
             (ArithOp::Sub, Value::Set(_), _) | (ArithOp::Sub, _, Value::Set(_)) => {
                 builtins::sets::difference(lhs, rhs, lhs_value, rhs_value)
@@ -904,13 +900,23 @@ impl<'source> Interpreter<'source> {
     ) -> Result<Value> {
         let mut args = vec![];
         for p in params {
-            args.push(self.eval_expr(p)?);
+            match self.eval_expr(p)? {
+                // If any argument is undefined, then the call is undefined.
+                Value::Undefined => return Ok(Value::Undefined),
+                p => args.push(p),
+            }
+        }
+
+        let cache = builtins::must_cache(name.as_str());
+        if let Some(name) = &cache {
+            if let Some(v) = self.builtins_cache.get(&(name, args.clone())) {
+                return Ok(v.clone());
+            }
         }
 
-        let is_randn = name == "rand.intn";
         let v = builtin(span, &params[..], &args[..])?;
-        if is_randn {
-            self.intns.insert(args, v.clone());
+        if let Some(name) = cache {
+            self.builtins_cache.insert((name, args), v.clone());
         }
         Ok(v)
     }
diff --git a/src/parser.rs b/src/parser.rs
index 8072c4f..f9110db 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -524,6 +524,12 @@ impl<'source> Parser<'source> {
                 }
                 "(" if possible_fcn => {
                     self.next_token()?;
+                    if self.tok.1.text() == ")" {
+                        return Err(self
+                            .tok
+                            .1
+                            .error("at least one argument required for function calls"));
+                    }
                     let mut args = vec![self.parse_in_expr()?];
                     while self.tok.1.text() == "," {
                         self.next_token()?;
diff --git a/tests/interpreter/cases/builtins/numbers.yaml b/tests/interpreter/cases/builtins/numbers.yaml
index 955972b..b56f5f2 100644
--- a/tests/interpreter/cases/builtins/numbers.yaml
+++ b/tests/interpreter/cases/builtins/numbers.yaml
@@ -7,10 +7,15 @@ cases:
     modules:
       - |
         package test
-        x = abs(-9)
-    query: data.test.x
-    want_result: 9
-    
+        x = [abs(-9), abs(9), abs(-9.1), abs(9.1)]
+
+        # Undefined
+        y { false }
+        z = abs(y)
+    query: data.test
+    want_result:
+      x: [9, 9, 9.1, 9.1]
+
   - note: abs-extra-args
     data: {}
     modules:
@@ -19,7 +24,7 @@ cases:
         x = abs(-9, 10)
     query: data.test.x
     error: "`abs` expects 1 argument"
-  
+
   - note: abs-invalid-type
     data: {}
     modules:
@@ -34,10 +39,14 @@ cases:
     modules:
       - |
         package test
-        x = [ceil(9.1), ceil(-9.1)]
-    query: data.test.x
-    want_result: [10, -9]
-    
+        x = [ceil(9.1), ceil(-9.1), ceil(9), ceil(-9)]
+        # Undefined
+        y { false }
+        z = ceil(y)
+    query: data.test
+    want_result:
+      x: [10, -9, 9, -9]
+
   - note: ceil-extra-args
     data: {}
     modules:
@@ -46,7 +55,7 @@ cases:
         x = ceil(-9, 10)
     query: data.test.x
     error: "`ceil` expects 1 argument"
-  
+
   - note: ceil-invalid-type
     data: {}
     modules:
@@ -55,16 +64,20 @@ cases:
         x = ceil("-9")
     query: data.test.x
     error: "`ceil` expects numeric argument"
-    
+
   - note: floor
     data: {}
     modules:
       - |
         package test
-        x = [floor(9.1), floor(-9.1)]
-    query: data.test.x
-    want_result: [9, -10]
-    
+        x = [floor(9.1), floor(-9.1), floor(9), floor(-9)]
+        # Undefined
+        y { false }
+        z = floor(y)
+    query: data.test
+    want_result:
+      x: [9, -10, 9, -9]
+
   - note: floor-extra-args
     data: {}
     modules:
@@ -73,7 +86,7 @@ cases:
         x = floor(-9, 10)
     query: data.test.x
     error: "`floor` expects 1 argument"
-  
+
   - note: floor-invalid-type
     data: {}
     modules:
@@ -82,7 +95,7 @@ cases:
         x = floor("-9")
     query: data.test.x
     error: "`floor` expects numeric argument"
-    
+
   - note: numbers.range
     data: {}
     modules:
@@ -93,19 +106,24 @@ cases:
         r3 = numbers.range(-1, -5)
         r4 = numbers.range(-5, -1)
 
-        # Non-integer start and end result in Undefined.
-        r5 = numbers.range(1.01, 5)
-        r6 = numbers.range(1, 5.01)
-
         # Single item range
-        r7 = numbers.range(8, 8)
+        r5 = numbers.range(8, 8)
+
+        # Non-integer start and end result in Undefined.
+        r6 = numbers.range(1.01, 5)
+        r7 = numbers.range(1, 5.01)
+
+        y { false }
+        r8 = numbers.range(y, 10)
+        r9 = numbers.range(10, y)
+
     query: data.test
     want_result:
       r1: [1, 2, 3, 4, 5]
       r2: [5, 4, 3, 2, 1]
       r3: [-1, -2, -3, -4, -5]
       r4: [-5, -4, -3, -2, -1]
-      r7: [8]
+      r5: [8]
 
   - note: numbers.range-less-args
     data: {}
@@ -115,7 +133,7 @@ cases:
         x = numbers.range(1)
     query: data.test.x
     error: "`numbers.range` expects 2 arguments"
-    
+
   - note: numbers.range-more-args
     data: {}
     modules:
@@ -124,7 +142,7 @@ cases:
         x = numbers.range(1, 2, 3)
     query: data.test.x
     error: "`numbers.range` expects 2 arguments"
-    
+
   - note: numbers.range-invalid-start
     data: {}
     modules:
@@ -133,7 +151,7 @@ cases:
         x = numbers.range("1", 2)
     query: data.test.x
     error: "`numbers.range` expects numeric argument"
-    
+
   - note: numbers.range-invalid-end
     data: {}
     modules:
@@ -149,10 +167,15 @@ cases:
       - |
         package test
         x = [round(-9.4), round(-9.5), round(-9.6),
-             round(9.4), round(9.5), round(9.6)]
+             round(9.4), round(9.5), round(9.6),
+             round(8), round(-8)]
+
+        # Undefined
+        y { false }
+        z = round(y)
     query: data.test.x
-    want_result: [-9, -10, -10, 9, 10, 10]
-    
+    want_result: [-9, -10, -10, 9, 10, 10, 8, -8]
+
   - note: round-extra-args
     data: {}
     modules:
@@ -161,7 +184,7 @@ cases:
         x = round(-9, 10)
     query: data.test.x
     error: "`round` expects 1 argument"
-  
+
   - note: round-invalid-type
     data: {}
     modules:
@@ -171,15 +194,96 @@ cases:
     query: data.test.x
     error: "`round` expects numeric argument"
 
+  - note: rand.intn
+    data: {}
+    modules:
+      - |
+        package test
+        x = rand.intn("x", 50000)
+        y = rand.intn("y", 50000)
+        z = [p |
+           p := rand.intn("x", 50000)
+        ]
+        a = rand.intn("x", 25000)
+
+        results = [
+          x == z[0],
+          x != y,
+          x != a,
+          rand.intn("b", 0)
+        ]
+    query: data.test.results
+    want_result: [true, true, true, 0]
+
+  - note: rand.intn-undefined
+    data: {}
+    modules:
+      - |
+        package test
+        y { false }
+        r1 = rand.intn(y, 10)
+        r2 = rand.intn(10, y)
+        r3 = rand.intn("a", 10.3)
+    query: data.test
+    want_result: {}
+
+  - note: rand.intn-extra-args
+    data: {}
+    modules:
+      - |
+        package test
+        x = rand.intn("abc", 10, 11)
+    query: data.test.x
+    error: "`rand.intn` expects 2 arguments"
+
+  - note: rand.intn-less-args
+    data: {}
+    modules:
+      - |
+        package test
+        x = rand.intn("abc")
+    query: data.test.x
+    error: "`rand.intn` expects 2 arguments"
+
+  - note: rand.intn-invalid-type-1
+    data: {}
+    modules:
+      - |
+        package test
+        x = rand.intn(1, 2)
+    query: data.test.x
+    error: "`rand.intn` expects string argument"
+
+  - note: rand.intn-invalid-type-2
+    data: {}
+    modules:
+      - |
+        package test
+        x = rand.intn("a", "b")
+    query: data.test.x
+    error: "`rand.intn` expects numeric argument"
+
+
   - note: div
     data: {}
     modules:
       - |
         package test
+        import future.keywords.if
+
         x = 1/3
-    query: data.test.x
-    want_result:  0.3333333333333333
-    
+
+        # Undefined
+        y if false
+        a = 1 / 0
+        b = y / 1
+        c = 1 / y
+        d = 13.3 % 3
+        e = 13 % 3.1
+    query: data.test
+    want_result:
+      x: 0.3333333333333333
+
   - note: div-non-numeric
     data: {}
     modules:
@@ -188,36 +292,23 @@ cases:
         x = "1" / 9
     query: data.test.x
     error: "`div` expects numeric argument."
-    
-  - note: div-undefined
-    data: {}
-    modules:
-      - |
-        package test
-        import future.keywords.if
-        a if false
-        x = a / 1
-    query: data.test
-    want_result: {}
 
-  - note: div-by-zero
-    data: {}
-    modules:
-      - |
-        package test
-        x = 1/0
-    query: data.test
-    want_result: {}
-    
   - note: mod
     data: {}
     modules:
       - |
         package test
         x = 10%3
-    query: data.test.x
-    want_result:  1
-    
+
+        # Undefined
+        y { false }
+        a = 1 % 0
+        b = y % 1
+        c = 1 % y
+    query: data.test
+    want_result:
+      x : 1
+
   - note: mod-non-numeric
     data: {}
     modules:
@@ -226,14 +317,13 @@ cases:
         x = "1" % 9
     query: data.test.x
     error: "`mod` expects numeric argument."
-    
+
   - note: mod-undefined
     data: {}
     modules:
       - |
         package test
-        import future.keywords.if
-        a if false
+        a { false }
         x = a % 10
     query: data.test
     want_result: {}
@@ -246,7 +336,76 @@ cases:
         x = 1%0
     query: data.test
     want_result: {}
-    
+
+  - note: mul
+    data: {}
+    modules:
+      - |
+        package test
+        x = 3 * -1 * 4.5
+        # Undefined
+        y { false }
+        a = 1 * y
+        b = y * 1
+    query: data.test
+    want_result:
+      x: -13.5
+
+  - note: mul-non-numeric
+    data: {}
+    modules:
+      - |
+        package test
+        x = "1" * 9
+    query: data.test.x
+    error: "`mul` expects numeric argument."
+
+  - note: add
+    data: {}
+    modules:
+      - |
+        package test
+        x = 3 + -1 + 4.5
+        # Undefined
+        y { false }
+        a = 1 + y
+        b = y + 1
+    query: data.test
+    want_result:
+      x: 6.5
+
+  - note: add-non-numeric
+    data: {}
+    modules:
+      - |
+        package test
+        x = "1" + 9
+    query: data.test.x
+    error: "`add` expects numeric argument."
+
+  - note: sub
+    data: {}
+    modules:
+      - |
+        package test
+        x = 3 - -1 - 4.5
+        # Undefined
+        y { false }
+        a = 1 - y
+        b = y - 1
+    query: data.test
+    want_result:
+      x: -0.5
+
+  - note: sub-non-numeric
+    data: {}
+    modules:
+      - |
+        package test
+        x = "1" - 9
+    query: data.test.x
+    error: "`sub` expects numeric argument."
+
   - note: all
     data: {}
     modules:
@@ -256,3 +415,5 @@ cases:
     query: data.test
     want_result:
       x: 1.5
+
+  # TODO: Lockdown associativity and precedence of operators.