diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..d1a98fa
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,31 @@
+# Binaries
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+*.test
+
+# Build output
+bin/
+build/
+dist/
+
+# Go workspace
+*.out
+
+# Dependency directories (if any)
+vendor/
+
+# Go coverage files
+coverage.out
+
+# IDE: GoLand / IntelliJ IDEA
+.idea/
+*.iml
+out/
+
+# Logs and temp files
+*.log
+*.tmp
+.DS_Store
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..eee1c81
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,5 @@
+module c65gm
+
+go 1.25.1
+
+require github.com/armon/go-radix v1.0.0 // indirect
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..09f0f80
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,2 @@
+github.com/armon/go-radix v1.0.0 h1:F4z6KzEeeQIMeLFa97iZU6vupzoecKdU5TX24SNppXI=
+github.com/armon/go-radix v1.0.0/go.mod h1:ufUuZ+zHj4x4TnLV4JWEpy2hxWSpsRywHrMgIH9cCH8=
diff --git a/internal/preproc/definelist.go b/internal/preproc/definelist.go
new file mode 100644
index 0000000..d374cf4
--- /dev/null
+++ b/internal/preproc/definelist.go
@@ -0,0 +1,57 @@
+package preproc
+
+import (
+	"strings"
+	"unicode/utf8"
+
+	"github.com/armon/go-radix"
+)
+
+type DefineList struct {
+	tree *radix.Tree
+}
+
+func NewDefineList() *DefineList { return &DefineList{tree: radix.New()} }
+
+// Add inserts or updates a define->value mapping.
+func (d *DefineList) Add(define, value string) { d.tree.Insert(define, value) }
+
+// Delete removes a define. Returns true if it existed.
+func (d *DefineList) Delete(define string) bool {
+	_, ok := d.tree.Delete(define)
+	return ok
+}
+
+// Defined reports whether a define exists.
+func (d *DefineList) Defined(s string) bool {
+	_, ok := d.tree.Get(s)
+	return ok
+}
+
+// typed wrapper to avoid type assertions sprinkled in hot code
+func (d *DefineList) longestPrefixString(s string) (key string, val string, ok bool) {
+	k, v, ok := d.tree.LongestPrefix(s)
+	if !ok {
+		return "", "", false
+	}
+	return k, v.(string), true
+}
+
+// ReplaceDefines performs a single-pass, longest-prefix replacement.
+// Case-sensitive. Matches anywhere. No recursive re-expansion.
+func (d *DefineList) ReplaceDefines(s string) string {
+	var b strings.Builder
+	b.Grow(len(s)) // baseline; grows if expansions occur
+
+	for i := 0; i < len(s); {
+		if key, val, ok := d.longestPrefixString(s[i:]); ok && key != "" {
+			b.WriteString(val)
+			i += len(key) // advance by matched bytes (UTF-8-safe)
+			continue
+		}
+		_, size := utf8.DecodeRuneInString(s[i:])
+		b.WriteString(s[i : i+size])
+		i += size
+	}
+	return b.String()
+}
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..7ac9c86
--- /dev/null
+++ b/main.go
@@ -0,0 +1,31 @@
+package main
+
+import (
+	"c65gm/internal/preproc"
+	"fmt"
+)
+
+// put the DefineList type in another file (same package) or import it;
+// here I assume definelist.go is in its own package "definelist".
+// If it's in the same package main, just call New() directly.
+
+func main() {
+	defs := preproc.NewDefineList() // <-- important
+
+	defs.Add("ARRAY_MIN", "0")
+	defs.Add("ARRAY_MAX", "100")
+	defs.Add("ARRAY_MAX_PAX", "1000")
+	defs.Add("VERSION", "1.0")
+	defs.Add("π", "3.14159")
+
+	src := `for i := ARRAY_MIN to ARRAY_MAX_PAX do
+  writeln(VERSION);
+  writeln(ARRAY_MAX);
+  const PI = π;`
+
+	fmt.Println("Original:\n" + src)
+	fmt.Println("\nReplaced:\n" + defs.ReplaceDefines(src))
+	defs.Delete("ARRAY_MAX_PAX")
+	fmt.Println("\nReplaced (after .Delete):\n" + defs.ReplaceDefines(src))
+
+}