diff --git a/SLS_Python/sls_calc/__init__.py b/SLS_Python/sls_calc/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/SLS_Python/sls_calc/__main__.py b/SLS_Python/sls_calc/__main__.py
new file mode 100644
index 0000000..e8565f3
--- /dev/null
+++ b/SLS_Python/sls_calc/__main__.py
@@ -0,0 +1,305 @@
+#!/usr/bin/env python3
+"""
+HP-Style RPN Calculator using Stack Language Backend
+Implements classic HP calculator interface with stack display
+"""
+
+import tkinter as tk
+from tkinter import ttk, font as tkfont
+import sls_py
+
+class HPCalculator:
+    def __init__(self, root):
+        self.root = root
+        self.root.title("HP Stack Calculator")
+        self.root.geometry("450x650")
+        self.root.resizable(False, False)
+        
+        # Initialize interpreter
+        self.interp = sls_py.InterpreterState()
+        self.lexer = sls_py.LexerInfo()
+        
+        # Current entry buffer
+        self.entry_buffer = ""
+        self.is_new_entry = True
+        
+        # Set up UI
+        self.setup_styles()
+        self.create_widgets()
+        
+    def setup_styles(self):
+        """Configure custom styles for the calculator"""
+        style = ttk.Style()
+        style.theme_use('clam')
+        
+        # Configure button styles
+        style.configure('Calc.TButton',
+                       font=('Arial', 12, 'bold'),
+                       padding=10)
+        style.configure('Display.TLabel',
+                       font=('Courier', 14),
+                       background='#f0f0f0',
+                       relief='sunken',
+                       padding=5)
+        
+    def create_widgets(self):
+        """Create all calculator widgets"""
+        # Main container
+        main_frame = ttk.Frame(self.root, padding="10")
+        main_frame.grid(row=0, column=0, sticky=(tk.W, tk.E, tk.N, tk.S))
+        
+        # Stack display (shows 4 stack levels like classic HP)
+        self.create_stack_display(main_frame)
+        
+        # Entry display
+        self.create_entry_display(main_frame)
+        
+        # Button grid
+        self.create_button_grid(main_frame)
+        
+    def create_stack_display(self, parent):
+        """Create the 4-level stack display"""
+        stack_frame = ttk.LabelFrame(parent, text="Stack", padding="5")
+        stack_frame.grid(row=0, column=0, columnspan=4, sticky=(tk.W, tk.E), pady=(0, 10))
+        
+        self.stack_labels = []
+        for i in range(4):
+            label = ttk.Label(stack_frame,
+                            text=f"T{3-i}: 0",
+                            style='Display.TLabel',
+                            anchor='e',
+                            width=30)
+            label.grid(row=i, column=0, sticky=(tk.W, tk.E), pady=2)
+            self.stack_labels.append(label)
+            
+    def create_entry_display(self, parent):
+        """Create the current entry display"""
+        entry_frame = ttk.Frame(parent)
+        entry_frame.grid(row=1, column=0, columnspan=4, sticky=(tk.W, tk.E), pady=(0, 10))
+        
+        self.entry_label = ttk.Label(entry_frame,
+                                     text="0",
+                                     style='Display.TLabel',
+                                     anchor='e',
+                                     font=('Courier', 16, 'bold'))
+        self.entry_label.grid(row=0, column=0, sticky=(tk.W, tk.E))
+        
+    def create_button_grid(self, parent):
+        """Create the calculator button grid"""
+        buttons = [
+            # Row 1: Stack operations
+            [('√', self.sqrt), ('x²', self.square), ('1/x', self.reciprocal), ('÷', self.divide)],
+            # Row 2: Numbers and operations
+            [('7', lambda: self.digit('7')), ('8', lambda: self.digit('8')), 
+             ('9', lambda: self.digit('9')), ('×', self.multiply)],
+            # Row 3
+            [('4', lambda: self.digit('4')), ('5', lambda: self.digit('5')), 
+             ('6', lambda: self.digit('6')), ('−', self.subtract)],
+            # Row 4
+            [('1', lambda: self.digit('1')), ('2', lambda: self.digit('2')), 
+             ('3', lambda: self.digit('3')), ('+', self.add)],
+            # Row 5
+            [('0', lambda: self.digit('0')), ('.', self.decimal), 
+             ('±', self.negate), ('ENTER', self.enter)],
+            # Row 6: Special functions
+            [('SWAP', self.swap), ('DROP', self.drop), 
+             ('DUP', self.dup), ('CLR', self.clear)]
+        ]
+        
+        for row_idx, row in enumerate(buttons, start=2):
+            for col_idx, (text, command) in enumerate(row):
+                btn = ttk.Button(parent,
+                               text=text,
+                               command=command,
+                               style='Calc.TButton',
+                               width=8)
+                btn.grid(row=row_idx, column=col_idx, padx=2, pady=2, sticky=(tk.W, tk.E, tk.N, tk.S))
+        
+        # Keyboard bindings
+        self.root.bind('<Return>', lambda e: self.enter())
+        self.root.bind('<KP_Enter>', lambda e: self.enter())
+        for i in range(10):
+            self.root.bind(str(i), lambda e, n=str(i): self.digit(n))
+        self.root.bind('.', lambda e: self.decimal())
+        self.root.bind('+', lambda e: self.add())
+        self.root.bind('-', lambda e: self.subtract())
+        self.root.bind('*', lambda e: self.multiply())
+        self.root.bind('/', lambda e: self.divide())
+        self.root.bind('<Escape>', lambda e: self.clear())
+        
+    def digit(self, d):
+        """Handle digit button press"""
+        if self.is_new_entry:
+            self.entry_buffer = d
+            self.is_new_entry = False
+        else:
+            self.entry_buffer += d
+        self.update_entry_display()
+        
+    def decimal(self):
+        """Handle decimal point"""
+        if self.is_new_entry:
+            self.entry_buffer = "0."
+            self.is_new_entry = False
+        elif '.' not in self.entry_buffer:
+            self.entry_buffer += '.'
+        self.update_entry_display()
+        
+    def negate(self):
+        """Toggle sign of current entry"""
+        if self.is_new_entry:
+            # Negate top of stack
+            self.execute_code("0 swap -")
+        else:
+            if self.entry_buffer.startswith('-'):
+                self.entry_buffer = self.entry_buffer[1:]
+            else:
+                self.entry_buffer = '-' + self.entry_buffer
+        self.update_entry_display()
+        
+    def enter(self):
+        """Push current entry onto stack"""
+        if self.entry_buffer:
+            self.execute_code(self.entry_buffer)
+            self.entry_buffer = ""
+            self.is_new_entry = True
+        self.update_displays()
+        
+    def add(self):
+        """Addition operation"""
+        self.enter()
+        self.execute_code("+")
+        self.update_displays()
+        
+    def subtract(self):
+        """Subtraction operation"""
+        self.enter()
+        self.execute_code("-")
+        self.update_displays()
+        
+    def multiply(self):
+        """Multiplication operation"""
+        self.enter()
+        self.execute_code("*")
+        self.update_displays()
+        
+    def divide(self):
+        """Division operation"""
+        self.enter()
+        self.execute_code("/")
+        self.update_displays()
+        
+    def sqrt(self):
+        """Square root operation"""
+        self.enter()
+        self.execute_code("sqrt")
+        self.update_displays()
+        
+    def square(self):
+        """Square operation"""
+        self.enter()
+        self.execute_code("dup *")
+        self.update_displays()
+        
+    def reciprocal(self):
+        """Reciprocal (1/x) operation"""
+        self.enter()
+        self.execute_code("1 swap /")
+        self.update_displays()
+        
+    def swap(self):
+        """Swap top two stack items"""
+        self.enter()
+        self.execute_code("swap")
+        self.update_displays()
+        
+    def drop(self):
+        """Drop top stack item"""
+        self.enter()
+        self.execute_code("drop")
+        self.update_displays()
+        
+    def dup(self):
+        """Duplicate top stack item"""
+        self.enter()
+        self.execute_code("dup")
+        self.update_displays()
+        
+    def clear(self):
+        """Clear entry and stack"""
+        self.entry_buffer = ""
+        self.is_new_entry = True
+        # Clear the stack
+        while len(self.interp.stack) > 0:
+            self.interp.pop()
+        self.update_displays()
+        
+    def execute_code(self, code):
+        """Execute stack language code"""
+        try:
+            self.lexer.source_code = code
+            self.lexer.pos = 0
+            self.lexer.column = 1
+            self.lexer.line = 1
+            
+            tokens = sls_py.lexical_analysis(self.lexer)
+            
+            for token in tokens:
+                if token.type == sls_py.TokenType.EOF:
+                    break
+                if not self.interp.execute(token):
+                    print(f"Error executing: {code}")
+                    break
+        except Exception as e:
+            print(f"Exception: {e}")
+            
+    def update_entry_display(self):
+        """Update the entry display"""
+        display_text = self.entry_buffer if self.entry_buffer else "0"
+        self.entry_label.config(text=display_text)
+        
+    def update_displays(self):
+        """Update both entry and stack displays"""
+        self.update_entry_display()
+        self.update_stack_display()
+        
+    def update_stack_display(self):
+        """Update the 4-level stack display"""
+        stack_size = len(self.interp.stack)
+        
+        for i in range(4):
+            level = 3 - i  # T3, T2, T1, T0
+            if stack_size > level:
+                entry = self.interp.stack[-(level + 1)]
+                value_str = self.format_stack_entry(entry)
+                self.stack_labels[i].config(text=f"T{level}: {value_str}")
+            else:
+                self.stack_labels[i].config(text=f"T{level}: 0")
+                
+    def format_stack_entry(self, entry):
+        """Format a stack entry for display"""
+        if entry.type == sls_py.StackType.I64:
+            return str(entry.value)
+        elif entry.type == sls_py.StackType.DOUBLE:
+            val = entry.value
+            # Format with appropriate precision
+            if abs(val) < 1e-10 and val != 0:
+                return f"{val:.6e}"
+            elif abs(val) > 1e10:
+                return f"{val:.6e}"
+            else:
+                return f"{val:.10g}"
+        elif entry.type == sls_py.StackType.BOOLEAN:
+            return str(entry.value)
+        else:
+            return str(entry.value)
+
+
+def main():
+    root = tk.Tk()
+    app = HPCalculator(root)
+    root.mainloop()
+
+
+if __name__ == "__main__":
+    main()