diff --git a/README.md b/README.md
index 53d447e..3be25e9 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,19 @@
 # gbso
 
 An implementation of the stochastic superoptimization algorithm [STOKE](docs/asplos13.pdf) for the Sharp LR35902 (also known as GB-Z80).
+
+## Setup
+
+Make sure you have [Poetry](https://python-poetry.org/) and Python 3.9+ installed. Then run:
+
+```
+$ poetry install
+```
+
+## Usage
+
+Input programs are specified as a list of CPU instructions. For a list of instructions available to the processor, take a look at [insn.py](/gbso/cpu/insn.py) or a [standard reference](https://gbdev.io/pandocs/CPU_Instruction_Set.html).
+
+Once you have the program specified, call `gbso.optimize.optimize` on the program. A superoptimized, behaviorally equivalent program will be returned. Additional parameters are available to the optimization function to control search.
+
+For a comprehensive example, see [ex.py](/ex.py).
diff --git a/ex.py b/ex.py
index 3b4d937..d2d4476 100644
--- a/ex.py
+++ b/ex.py
@@ -1,8 +1,16 @@
 from gbso.cpu.insn import *
 from gbso.cpu.regs import R8, R16
 
-from gbso.optimize import optimize
+from gbso.optimize import (
+    DEFAULT_PROB_INSN,
+    DEFAULT_PROB_INSN_UNUSED,
+    DEFAULT_PROB_OPCODE,
+    DEFAULT_PROB_OPERAND,
+    DEFAULT_PROB_SWAP,
+    optimize,
+)
 from gbso.program.program import Program
+from gbso.program.mutate import mutate_program
 
 rLCDC = 0xFF40
 rIE = 0xFFFF
@@ -10,8 +18,8 @@ rIE = 0xFFFF
 IEF_VBLANK = 1 << 0
 
 prgm = Program(
-    outputs=set([R8.A]),
     insns=[
+        LD_R_N8(R8.A, 12),
         LD_RR_NN(R16.HL, rLCDC),
         RES_N_HL(7),
         RES_N_HL(2),
@@ -20,6 +28,16 @@ prgm = Program(
     ],
 )
 
-prgm_opt = optimize(prgm)
-for insn in prgm_opt.insns:
-    print(insn.pretty())
+optimized_prgm = optimize(prgm)
+optimized_prgm.display()
+
+for _ in range(10):
+    prgm = mutate_program(
+        prgm,
+        prob_opcode=DEFAULT_PROB_OPCODE,
+        prob_operand=DEFAULT_PROB_OPERAND,
+        prob_swap=DEFAULT_PROB_SWAP,
+        prob_insn=DEFAULT_PROB_INSN,
+        prob_insn_unused=DEFAULT_PROB_INSN_UNUSED,
+    )
+    prgm.display()
diff --git a/gbso/program/mutate.py b/gbso/program/mutate.py
index d59b63c..0b87bc1 100644
--- a/gbso/program/mutate.py
+++ b/gbso/program/mutate.py
@@ -3,7 +3,7 @@ from gbso.cpu.regs import R16, R16_WITHOUT_SP, R8
 from random import choice, randint, random, randrange
 from typing import List, Tuple, Type, TypeVar, Union
 
-from gbso.cpu.insn import ALL_INSN_CLASSES, Insn, Operand, UNUSED
+from gbso.cpu.insn import ALL_INSN_CLASSES, Insn, Operand, UNUSED, get_signature_class
 from gbso.program.program import Program
 
 
@@ -31,20 +31,30 @@ def mutate_program(
         ]
     )
     new_prgm = prgm
+    print("======")
     if mutation == Mutation.OPCODE:
+        print("OPCODE")
         new_prgm = mutate_opcode(prgm)
     elif mutation == Mutation.OPERAND:
+        print("OPERAND")
         new_prgm = mutate_operand(prgm)
     elif mutation == Mutation.SWAP:
+        print("SWAP")
         new_prgm = mutate_swap(prgm)
     elif mutation == Mutation.INSTRUCTION:
+        print("INSN")
         new_prgm = mutate_insn(prgm, prob_insn_unused)
     return new_prgm
 
 
-# TODO: Implement
 def mutate_opcode(prgm: Program) -> Program:
-    return prgm
+    i = randrange(len(prgm.insns))
+    next_insn_cls = choice(get_signature_class(prgm.insns[i]))
+
+    insns = prgm.insns.copy()
+    insns[i] = next_insn_cls(*vars(prgm.insns[i]).values())
+
+    return Program(insns=insns)
 
 
 # TODO: Implement
diff --git a/gbso/program/program.py b/gbso/program/program.py
index 0b39b1b..588145a 100644
--- a/gbso/program/program.py
+++ b/gbso/program/program.py
@@ -19,3 +19,8 @@ class Program:
 
     def perf(self) -> int:
         return sum([insn.cycles() for insn in self.insns])
+
+    def display(self) -> None:
+        print("=" * 10)
+        for insn in self.insns:
+            print(insn.pretty())