diff --git a/src/prolog/builtins/analysingAndConstructionOperators.kt b/src/prolog/builtins/analysingAndConstructionOperators.kt
index ba589ef..f2a9079 100644
--- a/src/prolog/builtins/analysingAndConstructionOperators.kt
+++ b/src/prolog/builtins/analysingAndConstructionOperators.kt
@@ -3,56 +3,43 @@ package prolog.builtins
 import prolog.Answers
 import prolog.Substitutions
 import prolog.ast.arithmetic.Integer
-import prolog.ast.terms.AnonymousVariable
-import prolog.ast.terms.Atom
-import prolog.ast.terms.Head
-import prolog.ast.terms.Structure
-import prolog.ast.terms.Term
-import prolog.ast.terms.Variable
-import prolog.logic.applySubstitution
-import prolog.logic.atomic
-import prolog.logic.nonvariable
-import prolog.logic.unifyLazy
-import prolog.logic.variable
+import prolog.ast.terms.*
+import prolog.logic.*
+import java.util.Locale
+import java.util.Locale.getDefault
 
+/**
+ * [True] when [Term] is a term with [FunctorInfo] Name/Arity.
+ *
+ * If Term is a [Variable] it is unified with a new term whose arguments are all different variables.
+ * If Term is [atomic], Arity will be unified with the integer 0, and Name will be unified with Term.
+ */
 class Functor(private val term: Term, private val functorName: Term, private val functorArity: Term) :
     Structure(Atom("functor"), listOf(term, functorName, functorArity)) {
     override fun satisfy(subs: Substitutions): Answers {
-        if (nonvariable(term, subs)) {
-            val t = applySubstitution(term, subs) as Head
+        return when {
+            nonvariable(term, subs) -> {
+                val t = applySubstitution(term, subs) as Head
 
-            return Conjunction(
-                Unify(t.functor.arity, functorArity),
-                Unify(t.functor.name, functorName)
-            ).satisfy(subs)
-        }
-
-        if (variable(term, subs)) {
-            require(atomic(functorName, subs) && atomic(functorArity, subs)) {
-                "Arguments are not sufficiently instantiated"
+                Conjunction(
+                    Unify(t.functor.arity, functorArity),
+                    Unify(t.functor.name, functorName)
+                ).satisfy(subs)
             }
 
-            val name = applySubstitution(functorName, subs) as Atom
-            val arity = applySubstitution(functorArity, subs) as Integer
-
-            val result = if (arity.value == 0) {
-                functorName
-            } else {
-                val argList = mutableListOf<Term>()
-                for (i in 1..arity.value) {
-                    argList.add(AnonymousVariable.create())
+            variable(term, subs) -> {
+                require(atomic(functorName, subs) && atomic(functorArity, subs)) {
+                    "Arguments are not sufficiently instantiated"
                 }
-                Structure(name, argList)
+
+                val name = applySubstitution(functorName, subs) as Atom
+                val arity = applySubstitution(functorArity, subs) as Integer
+                val result = Structure(name, List(arity.value) { AnonymousVariable.create() })
+                sequenceOf(Result.success(mapOf(term to result)))
             }
 
-            if (nonvariable(term, subs)) {
-                return unifyLazy(term, result, subs)
-            }
-
-            return sequenceOf(Result.success(mapOf(term to result)))
+            else -> throw IllegalStateException()
         }
-
-        throw IllegalStateException()
     }
 
     override fun applySubstitution(subs: Substitutions): Functor = Functor(
@@ -60,6 +47,51 @@ class Functor(private val term: Term, private val functorName: Term, private val
         functorName.applySubstitution(subs),
         functorArity.applySubstitution(subs)
     )
-
-    override fun toString(): String = "functor($term, $functorName, $functorArity)"
+}
+
+class Arg(private val arg: Term, private val term: Term, private val value: Term) :
+    Structure(Atom("arg"), listOf(arg, term, value)) {
+    override fun satisfy(subs: Substitutions): Answers = sequence {
+        require(nonvariable(term, subs)) { "Arguments are not sufficiently instantiated" }
+        require(compound(term, subs)) {
+            val expected = CompoundTerm::class.simpleName?.lowercase()
+            val actual = term::class.simpleName?.lowercase()
+            "Type error: `$expected' expected, found `$term' ($actual)"
+        }
+
+        val t = applySubstitution(term, subs) as Structure
+
+        when {
+            variable(arg, subs) -> {
+                // Value will be unified with the successive arguments of term.
+                // On successful unification, arg is unified with the argument number.
+                // Backtracking yields alternative solutions.
+                var count = 0
+                for (argument in t.arguments) {
+                    unifyLazy(value, argument, subs).forEach { result ->
+                        result.map {
+                            val sub = arg to Integer(count + 1)
+                            yield(Result.success(it + sub))
+                        }
+                    }
+                    count++
+                }
+
+            }
+
+            else -> {
+                val a = applySubstitution(arg, subs) as Integer
+
+                require(0 <= a.value) { "Domain error: not_less_than_zero" }
+
+                // Fail silently if the argument is out of bounds
+                if (0 == a.value || t.arguments.size < a.value) {
+                    return@sequence
+                }
+
+                val argument = t.arguments[a.value - 1]
+                yieldAll(unifyLazy(argument, value, subs))
+            }
+        }
+    }
 }
diff --git a/tests/prolog/builtins/AnalysingAndConstructionOperatorsTests.kt b/tests/prolog/builtins/AnalysingAndConstructionOperatorsTests.kt
index 6e91bfc..8c2e28d 100644
--- a/tests/prolog/builtins/AnalysingAndConstructionOperatorsTests.kt
+++ b/tests/prolog/builtins/AnalysingAndConstructionOperatorsTests.kt
@@ -4,6 +4,7 @@ import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertInstanceOf
 import org.junit.jupiter.api.Assertions.assertTrue
 import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertThrows
 import prolog.ast.arithmetic.Integer
 import prolog.ast.terms.AnonymousVariable
 import prolog.ast.terms.Atom
@@ -88,4 +89,148 @@ class AnalysingAndConstructionOperatorsTests {
 
         assertTrue(result.isEmpty(), "Expected no results")
     }
+
+    @Test
+    fun `functor(X, Y, 1)`() {
+        val functor = Functor(Variable("X"), Variable("Y"), Integer(1))
+
+        val exception = assertThrows<Exception> {
+            functor.satisfy(emptyMap()).toList()
+        }
+        assertEquals("Arguments are not sufficiently instantiated", exception.message)
+    }
+
+    @Test
+    fun `arg without variables`() {
+        val arg = Arg(
+            Integer(1),
+            Structure(Atom("f"), listOf(Atom("a"), Atom("b"), Atom("c"))),
+            Atom("a")
+        )
+
+        val result = arg.satisfy(emptyMap()).toList()
+
+        assertEquals(1, result.size, "Expected 1 result")
+        assertTrue(result[0].isSuccess, "Expected success")
+        val subs = result[0].getOrNull()!!
+        assertTrue(subs.isEmpty(), "Expected no substitutions")
+    }
+
+    @Test
+    fun `arg with variable value`() {
+        val arg = Arg(
+            Integer(1),
+            Structure(Atom("f"), listOf(Atom("a"), Atom("b"), Atom("c"))),
+            Variable("Term")
+        )
+
+        val result = arg.satisfy(emptyMap()).toList()
+
+        assertEquals(1, result.size, "Expected 1 result")
+        assertTrue(result[0].isSuccess, "Expected success")
+        val subs = result[0].getOrNull()!!
+        assertEquals(1, subs.size, "Expected 1 substitution")
+        assertEquals(Atom("a"), subs[Variable("Term")])
+    }
+
+    @Test
+    fun `arg with variable arg`() {
+        val arguments = listOf(Atom("a"), Atom("b"), Atom("c"))
+        for (i in arguments.indices) {
+            val arg = Arg(
+                Variable("Arg"),
+                Structure(Atom("f"), arguments),
+                arguments[i]
+            )
+
+            val result = arg.satisfy(emptyMap()).toList()
+
+            assertEquals(1, result.size, "Expected 1 result for arg $i")
+            assertTrue(result[0].isSuccess, "Expected success for arg $i")
+            val subs = result[0].getOrNull()!!
+            assertEquals(1, subs.size, "Expected 1 substitution for arg $i")
+            assertEquals(Integer(i + 1), subs[Variable("Arg")], "Expected arg to be $i + 1")
+        }
+    }
+
+    @Test
+    fun `arg with backtracking`() {
+        val arg = Arg(
+            Variable("Position"),
+            Structure(Atom("f"), listOf(Atom("a"), Atom("b"), Atom("c"))),
+            Variable("Term")
+        )
+
+        val result = arg.satisfy(emptyMap()).toList()
+
+        assertEquals(3, result.size, "Expected 3 results")
+
+        assertTrue(result[0].isSuccess, "Expected success")
+        val subs1 = result[0].getOrNull()!!
+        assertEquals(2, subs1.size, "Expected 2 substitutions")
+        assertEquals(Integer(1), subs1[Variable("Position")])
+        assertEquals(Atom("a"), subs1[Variable("Term")])
+
+        assertTrue(result[1].isSuccess, "Expected success")
+        val subs2 = result[1].getOrNull()!!
+        assertEquals(2, subs2.size, "Expected 2 substitutions")
+        assertEquals(Integer(2), subs2[Variable("Position")])
+        assertEquals(Atom("b"), subs2[Variable("Term")])
+
+        assertTrue(result[2].isSuccess, "Expected success")
+        val subs3 = result[2].getOrNull()!!
+        assertEquals(2, subs3.size, "Expected 2 substitutions")
+        assertEquals(Integer(3), subs3[Variable("Position")])
+        assertEquals(Atom("c"), subs3[Variable("Term")])
+    }
+
+    @Test
+    fun `arg raises error if Arg is not compound`() {
+        val arg = Arg(Integer(1), Atom("foo"), Variable("X"))
+
+        val exception = assertThrows<IllegalArgumentException> {
+            arg.satisfy(emptyMap()).toList()
+        }
+        assertEquals("Type error: `structure' expected, found `foo' (atom)", exception.message)
+    }
+
+    @Test
+    fun `arg fails silently if arg = 0`() {
+        val arg = Arg(
+            Integer(0),
+            Structure(Atom("f"), listOf(Atom("a"), Atom("b"), Atom("c"))),
+            Variable("Term")
+        )
+
+        val result = arg.satisfy(emptyMap()).toList()
+
+        assertTrue(result.isEmpty(), "Expected no results")
+    }
+
+    @Test
+    fun `arg fails silently if arg gt arity`() {
+        val arg = Arg(
+            Integer(4),
+            Structure(Atom("f"), listOf(Atom("a"), Atom("b"), Atom("c"))),
+            Variable("Term")
+        )
+
+        val result = arg.satisfy(emptyMap()).toList()
+
+        assertTrue(result.isEmpty(), "Expected no results")
+    }
+
+    @Test
+    fun `arg raises error if arg lt 0`() {
+        val arg = Arg(
+            Integer(-1),
+            Structure(Atom("f"), listOf(Atom("a"), Atom("b"), Atom("c"))),
+            Variable("Term")
+        )
+
+        val exception = assertThrows<IllegalArgumentException> {
+            arg.satisfy(emptyMap()).toList()
+        }
+        assertEquals("Domain error: not_less_than_zero", exception.message)
+    }
 }
\ No newline at end of file