diff --git a/src/interpreter/Preprocessor.kt b/src/interpreter/Preprocessor.kt
index b6e7656..d10b605 100644
--- a/src/interpreter/Preprocessor.kt
+++ b/src/interpreter/Preprocessor.kt
@@ -1,4 +1,93 @@
 package interpreter
 
-class Preprocessor {
+import io.Logger
+import prolog.ast.logic.Clause
+import prolog.ast.logic.Fact
+import prolog.ast.logic.LogicOperand
+import prolog.ast.logic.Rule
+import prolog.ast.terms.Atom
+import prolog.ast.terms.Body
+import prolog.ast.terms.Goal
+import prolog.ast.terms.Head
+import prolog.ast.terms.Structure
+import prolog.ast.terms.Term
+import prolog.builtins.Conjunction
+import prolog.builtins.Cut
+import prolog.builtins.Disjunction
+import prolog.builtins.Fail
+import prolog.builtins.False
+import prolog.builtins.Not
+import prolog.builtins.Query
+import prolog.builtins.True
+
+/**
+ * Preprocessor for Prolog
+ *
+ * This class preprocesses Prolog code and applies various transformations such as recognizing builtins.
+ */
+open class Preprocessor {
+    /**
+     * Preprocesses the input Prolog code.
+     *
+     * @param input The already parsed Prolog code as a list of clauses.
+     * @return The preprocessed Prolog code as a list of clauses.
+     */
+    fun preprocess(input: List<Clause>): List<Clause> {
+        return input.map { preprocess(it) }
+    }
+
+    fun preprocess(input: Query): Query {
+        return Query(preprocess(input.query) as Goal)
+    }
+
+    private fun preprocess(clause: Clause): Clause {
+        return when (clause) {
+            is Fact -> {
+                Fact(preprocess(clause.head) as Head)
+            }
+            is Rule -> {
+                Rule(
+                    preprocess(clause.head) as Head,
+                    preprocess(clause.body as Term) as Body
+                )
+            }
+            else -> clause
+        }
+    }
+
+    protected open fun preprocess(term: Term): Term {
+        val prepped = when (term) {
+            Atom("true") -> True
+            Structure(Atom("true"), emptyList()) -> True
+            Atom("false") -> False
+            Structure(Atom("false"), emptyList()) -> False
+            Atom("fail") -> Fail
+            Structure(Atom("fail"), emptyList()) -> Fail
+            Atom("!") -> Cut()
+            Structure(Atom("!"), emptyList()) -> Cut()
+            else -> {
+                when {
+                    term is Structure && term.functor == ",/2" -> {
+                        val args = term.arguments.map { preprocess(it) }
+                        Conjunction(args[0] as LogicOperand, args[1] as LogicOperand)
+                    }
+                    term is Structure && term.functor == ";/2" -> {
+                        val args = term.arguments.map { preprocess(it) }
+                        Disjunction(args[0] as LogicOperand, args[1] as LogicOperand)
+                    }
+                    term is Structure && term.functor == "\\+/1" -> {
+                        val args = term.arguments.map { preprocess(it) }
+                        Not(args[0] as Goal)
+                    }
+                    else -> term
+                }
+            }
+        }
+
+        if (prepped != term || prepped::class != term::class) {
+            Logger.debug("Preprocessed term: $term -> $prepped (${prepped::class})")
+        }
+
+        return prepped
+    }
 }
\ No newline at end of file
diff --git a/src/prolog/builtins/arithmeticOperators.kt b/src/prolog/builtins/arithmeticOperators.kt
index 2463313..7931e7d 100644
--- a/src/prolog/builtins/arithmeticOperators.kt
+++ b/src/prolog/builtins/arithmeticOperators.kt
@@ -152,8 +152,8 @@ class Between(private val expr1: Expression, private val expr2: Expression, priv
 
         require(e1.to is Integer && e2.to is Integer) { "Arguments must be integers" }
 
-        val v1 = e1.to as Integer
-        val v2 = e2.to as Integer
+        val v1 = e1.to
+        val v2 = e2.to
 
         return if (variable(e3.to, subs)) {
             between(v1, v2, e3.to as Variable).map { answer ->
diff --git a/src/repl/Repl.kt b/src/repl/Repl.kt
index 699ab86..52cce29 100644
--- a/src/repl/Repl.kt
+++ b/src/repl/Repl.kt
@@ -1,5 +1,6 @@
 package repl
 
+import interpreter.Preprocessor
 import io.Logger
 import io.Terminal
 import parser.ReplParser
@@ -9,6 +10,7 @@ import prolog.Answers
 class Repl {
     private val io = Terminal()
     private val parser = ReplParser()
+    private val preprocessor = Preprocessor()
 
     fun start() {
         io.say("Prolog REPL. Type '^D' to quit.\n")
@@ -23,7 +25,8 @@ class Repl {
 
     fun query(): Answers {
         val queryString = io.prompt("?-", { "" })
-        val query = parser.parse(queryString)
+        val simpleQuery = parser.parse(queryString)
+        val query = preprocessor.preprocess(simpleQuery)
         return query.satisfy(emptyMap())
     }
 
@@ -46,7 +49,10 @@ class Repl {
                 }
 
                 when (command) {
-                    ";" -> previous = iterator.next()
+                    ";" -> {
+                        previous = iterator.next()
+                        io.say(prettyPrint(previous))
+                    }
                     "a" -> return
                     "." -> return
                     "h" -> {
@@ -55,8 +61,6 @@ class Repl {
                     }
                 }
             }
-
-            io.say(prettyPrint(previous))
         }
 
         io.say("\n")
diff --git a/tests/interpreter/PreprocessorTests.kt b/tests/interpreter/PreprocessorTests.kt
new file mode 100644
index 0000000..a97755a
--- /dev/null
+++ b/tests/interpreter/PreprocessorTests.kt
@@ -0,0 +1,156 @@
+package interpreter
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Nested
+import org.junit.jupiter.api.Test
+import prolog.ast.terms.Atom
+import prolog.ast.terms.CompoundTerm
+import prolog.ast.terms.Term
+import prolog.builtins.Conjunction
+import prolog.builtins.Disjunction
+import prolog.builtins.Cut
+import prolog.builtins.Fail
+import prolog.builtins.True
+import prolog.builtins.Not
+
+class PreprocessorTests {
+    class OpenPreprocessor : Preprocessor() {
+        public override fun preprocess(input: Term): Term {
+            return super.preprocess(input)
+        }
+    }
+
+    @Nested
+    class `Arithmetic operators` {
+        @Test
+        fun `evaluates to different`() {
+            assertEquals(1, 2)
+        }
+    }
+
+    @Nested
+    class `Control operators` {
+        private var preprocessor = OpenPreprocessor()
+
+        @Test
+        fun fail() {
+            val tests = mapOf(
+                Atom("fail") to Fail,
+                CompoundTerm(Atom("fail"), emptyList()) to Fail,
+                Atom("Fail") to Atom("Fail"),
+                CompoundTerm(Atom("Fail"), emptyList()) to CompoundTerm(Atom("Fail"), emptyList()),
+                CompoundTerm(Atom("fail"), listOf(Atom("a"))) to CompoundTerm(Atom("fail"), listOf(Atom("a"))),
+                CompoundTerm(Atom("fail"), listOf(Atom("fail"))) to CompoundTerm(Atom("fail"), listOf(Fail))
+            )
+
+            for ((input, expected) in tests) {
+                val result = preprocessor.preprocess(input)
+                assertEquals(expected, result, "Expected preprocessed")
+                assertEquals(expected::class, result::class, "Expected same class")
+            }
+        }
+
+        @Test
+        fun `true`() {
+            val tests = mapOf(
+                Atom("true") to True,
+                CompoundTerm(Atom("true"), emptyList()) to True,
+                Atom("True") to Atom("True"),
+                CompoundTerm(Atom("True"), emptyList()) to CompoundTerm(Atom("True"), emptyList()),
+                CompoundTerm(Atom("true"), listOf(Atom("a"))) to CompoundTerm(Atom("true"), listOf(Atom("a"))),
+                CompoundTerm(Atom("true"), listOf(Atom("true"))) to CompoundTerm(Atom("true"), listOf(True))
+            )
+
+            for ((input, expected) in tests) {
+                val result = preprocessor.preprocess(input)
+                assertEquals(expected, result, "Expected preprocessed")
+                assertEquals(expected::class, result::class, "Expected same class")
+            }
+        }
+
+        @Test
+        fun cut() {
+            val tests = mapOf(
+                Atom("!") to Cut(),
+                CompoundTerm(Atom("!"), emptyList()) to Cut(),
+                CompoundTerm(Atom("!"), listOf(Atom("a"))) to CompoundTerm(Atom("!"), listOf(Atom("a"))),
+                CompoundTerm(Atom("!"), listOf(Atom("!"))) to CompoundTerm(Atom("!"), listOf(Cut()))
+            )
+
+            for ((input, expected) in tests) {
+                val result = preprocessor.preprocess(input)
+                assertEquals(expected, result, "Expected preprocessed")
+                assertEquals(expected::class, result::class, "Expected same class")
+            }
+        }
+
+        @Test
+        fun conjunction() {
+            val tests = mapOf(
+                CompoundTerm(Atom(","), listOf(Atom("a"), Atom("b"))) to Conjunction(Atom("a"), Atom("b")),
+                CompoundTerm(Atom(","), listOf(Atom("a"), Atom("b"), Atom("c"))) to CompoundTerm(
+                    Atom(","),
+                    listOf(Atom("a"), Atom("b"), Atom("c"))
+                ),
+                // Nested conjunctions
+                CompoundTerm(
+                    Atom(","),
+                    listOf(Atom("a"), CompoundTerm(Atom(","), listOf(Atom("b"), Atom("c"))))
+                ) to Conjunction(Atom("a"), Conjunction(Atom("b"), Atom("c"))),
+            )
+
+            for ((input, expected) in tests) {
+                val result = preprocessor.preprocess(input)
+                assertEquals(expected, result, "Expected preprocessed")
+                assertEquals(expected::class, result::class, "Expected same class")
+            }
+        }
+
+        @Test
+        fun disjunction() {
+            val tests = mapOf(
+                CompoundTerm(Atom(";"), listOf(Atom("a"), Atom("b"))) to Disjunction(Atom("a"), Atom("b")),
+                CompoundTerm(Atom(";"), listOf(Atom("a"), Atom("b"), Atom("c"))) to CompoundTerm(
+                    Atom(";"),
+                    listOf(Atom("a"), Atom("b"), Atom("c"))
+                ),
+                // Nested disjunctions
+                CompoundTerm(
+                    Atom(";"),
+                    listOf(Atom("a"), CompoundTerm(Atom(";"), listOf(Atom("b"), Atom("c"))))
+                ) to Disjunction(Atom("a"), Disjunction(Atom("b"), Atom("c"))),
+            )
+
+            for ((input, expected) in tests) {
+                val result = preprocessor.preprocess(input)
+                assertEquals(expected, result, "Expected preprocessed")
+                assertEquals(expected::class, result::class, "Expected same class")
+            }
+        }
+
+        @Test
+        fun not() {
+            val tests = mapOf(
+                CompoundTerm(Atom("\\+"), listOf(Atom("a"))) to Not(Atom("a")),
+                CompoundTerm(Atom("\\+"), listOf(Atom("a"), Atom("b"))) to CompoundTerm(
+                    Atom("\\+"),
+                    listOf(Atom("a"), Atom("b"))
+                ),
+                // Nested not
+                CompoundTerm(
+                    Atom("foo"),
+                    listOf(
+                        Atom("bar"),
+                        CompoundTerm(Atom("\\+"), listOf(CompoundTerm(Atom("\\+"), listOf(Atom("baz")))))
+                    )
+                ) to CompoundTerm(Atom("foo"), listOf(Atom("bar"), Not(Not(Atom("baz"))))),
+            )
+
+            for ((input, expected) in tests) {
+                val result = preprocessor.preprocess(input)
+                assertEquals(expected, result, "Expected preprocessed")
+                assertEquals(expected::class, result::class, "Expected same class")
+            }
+        }
+    }
+}
diff --git a/tests/parser/OperatorParserTests.kt b/tests/parser/OperatorParserTests.kt
new file mode 100644
index 0000000..e15e89d
--- /dev/null
+++ b/tests/parser/OperatorParserTests.kt
@@ -0,0 +1,28 @@
+package parser
+
+import com.github.h0tk3y.betterParse.grammar.Grammar
+import com.github.h0tk3y.betterParse.grammar.parseToEnd
+import com.github.h0tk3y.betterParse.parser.Parser
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+import parser.grammars.TermsGrammar
+import prolog.ast.terms.Atom
+import prolog.ast.terms.Operator
+import prolog.ast.terms.Structure
+
+class OperatorParserTests {
+    class OperatorParser: TermsGrammar() {
+        override val rootParser: Parser<Operator> by operator
+    }
+
+    private var parser = OperatorParser() as Grammar<Operator>
+
+    @Test
+    fun `parse conjunction`() {
+        val input = "a, b"
+
+        val result = parser.parseToEnd(input)
+
+        assertEquals(Structure(Atom(","), listOf(Atom("a"), Atom("b"))), result, "Expected atom 'a, b'")
+    }
+}
\ No newline at end of file