diff --git a/docs/abstractions2.py b/docs/abstractions2.py
index a7cf946e5a..8ddf722cd5 100644
--- a/docs/abstractions2.py
+++ b/docs/abstractions2.py
@@ -101,6 +101,10 @@ print(sched[-1].ast)
 # run that schedule
 run_schedule(sched)
 
+# NOTE: UOps are no longer mutable, you have to fetch this from the becomes_map
+from tinygrad.ops import becomes_map
+out = becomes_map[out]
+
 # check the data out
 assert out.realized is not None and out.realized.as_buffer().cast('I')[0] == 5
 
diff --git a/test/unit/test_tensor_uop_representation.py b/test/unit/test_tensor_uop_representation.py
index bb5c2f596d..f0792acbeb 100644
--- a/test/unit/test_tensor_uop_representation.py
+++ b/test/unit/test_tensor_uop_representation.py
@@ -8,8 +8,6 @@ def is_pattern_uop(u:UOp, pat:UPat): assert pat.match(u, {}), f"{u}\nis not\n{pa
 def is_pattern(ten:Tensor, pat:UPat): is_pattern_uop(ten.lazydata, pat)
 
 class TestTensorMutates(unittest.TestCase):
-  # this fails because uops are mutating
-  @unittest.expectedFailure
   def test_mutate_add(self):
     a = Tensor([1,2,3])
     b = Tensor([4,5,6])
diff --git a/tinygrad/ops.py b/tinygrad/ops.py
index e6b5dc3e49..d06e57d873 100644
--- a/tinygrad/ops.py
+++ b/tinygrad/ops.py
@@ -231,6 +231,8 @@ buffers:weakref.WeakKeyDictionary[UOp, Buffer] = weakref.WeakKeyDictionary() # t
 all_metadata:weakref.WeakKeyDictionary[UOp, Metadata] = weakref.WeakKeyDictionary()
 forced_realize:weakref.WeakSet[UOp] = weakref.WeakSet()
 
+becomes_map: weakref.WeakKeyDictionary[UOp, UOp] = weakref.WeakKeyDictionary()
+
 # NOTE: this should be frozen, but frozen is slower
 @dataclass(eq=False, slots=True)
 class UOp(MathTrait, metaclass=UOpMetaClass):
@@ -471,12 +473,9 @@ class UOp(MathTrait, metaclass=UOpMetaClass):
   @property
   def forced_realize(self): return self in forced_realize
 
-  # *** danger zone ***
+  # *** less danger zone ***
 
-  # CAUTION: MUTABILITY!
-  def become(self, u:UOp):
-    del UOpMetaClass.ucache[(self.op, self.dtype, self.src, self.arg)]
-    self.op, self.dtype, self.src, self.arg = u.op, u.dtype, u.src, u.arg
+  def become(self, u:UOp): becomes_map[self] = u
 
   # *** uop movement ops ***
 
@@ -1205,7 +1204,9 @@ symbolic_simple = PatternMatcher([
   # NOTE: this can be wrong for loaded NaN
   (UPat.var("x") * 0, lambda x: x.const_like(float("nan") if isinstance(x.arg, float) and (math.isnan(x.arg) or math.isinf(x.arg)) else 0)),
   # ** constant folding **
-  (UPat(GroupOp.ALU, name="a", src=UPat((Ops.VCONST, Ops.CONST))), lambda a: a.const_like(exec_alu(a.op, a.dtype, [x.arg for x in a.src], False))),
+  # TODO: add const folding for Ops.THREEFRY
+  (UPat(GroupOp.ALU, name="a", src=UPat((Ops.VCONST, Ops.CONST))),
+   lambda a: a.const_like(exec_alu(a.op, a.dtype, [x.arg for x in a.src], False)) if a.op is not Ops.THREEFRY else None),
   # bool MUL is AND, ADD/MAX is OR. prevents other rules to rewrite bool ADD/MUL incorrectly
   (UPat.var('x', dtype=dtypes.bool) * UPat.var('y', dtype=dtypes.bool), lambda x,y: x&y),
   (UPat.var('x', dtype=dtypes.bool) + UPat.var('y', dtype=dtypes.bool), lambda x,y: x|y),
diff --git a/tinygrad/tensor.py b/tinygrad/tensor.py
index 5e77084e93..8392c15cd8 100644
--- a/tinygrad/tensor.py
+++ b/tinygrad/tensor.py
@@ -1,6 +1,6 @@
 # inspired by https://github.com/karpathy/micrograd/blob/master/micrograd/engine.py
 from __future__ import annotations
-import time, math, itertools, functools, struct, sys, inspect, pathlib, string, dataclasses, hashlib
+import time, math, itertools, functools, struct, sys, inspect, pathlib, string, dataclasses, hashlib, weakref
 from contextlib import ContextDecorator
 from typing import List, Tuple, Callable, Optional, ClassVar, Type, Union, Sequence, cast, get_args, Literal, TYPE_CHECKING, SupportsIndex
 from tinygrad.dtype import DType, DTypeLike, dtypes, ImageDType, ConstType, least_upper_float, least_upper_dtype, sum_acc_dtype, to_dtype, truncate
@@ -8,12 +8,17 @@ from tinygrad.helpers import argfix, make_tuple, flatten, prod, all_int, round_u
 from tinygrad.helpers import IMAGE, DEBUG, WINO, _METADATA, Metadata, TRACEMETA, ceildiv, fetch, polyN, unwrap
 from tinygrad.multi import MultiLazyBuffer
 from tinygrad.gradient import compute_gradient
-from tinygrad.ops import smax, smin, resolve, UOp, Ops, sint, Variable, SimpleMathTrait, identity_element
+from tinygrad.ops import smax, smin, resolve, UOp, Ops, sint, Variable, SimpleMathTrait, identity_element, becomes_map
 from tinygrad.device import Device, Buffer, BufferSpec
 from tinygrad.engine.realize import run_schedule
 from tinygrad.engine.memory import memory_planner
 from tinygrad.engine.schedule import ScheduleItem, create_schedule_with_vars
 
+# *** all in scope Tensors are here. this is the only way to get children ***
+# TODO: different "universes" for disconnected Tensors
+
+all_tensors: weakref.WeakSet[Tensor] = weakref.WeakSet()
+
 # **** start with two base classes, Tensor and Function ****
 
 class Function:
@@ -121,6 +126,11 @@ class Tensor(SimpleMathTrait):
   training: ClassVar[bool] = False
   no_grad: ClassVar[bool] = False
 
+  def __new__(cls, *args, **kwargs):
+    instance = super().__new__(cls)
+    all_tensors.add(instance)
+    return instance
+
   def __init__(self, data:Union[None, ConstType, bytes, List, Tuple, UOp, MultiLazyBuffer, 'np.ndarray', pathlib.Path],  # type: ignore [name-defined] # noqa: F821
                device:Optional[Union[str, tuple, list]]=None, dtype:Optional[DTypeLike]=None, requires_grad:Optional[bool]=None):
     if dtype is not None: dtype = to_dtype(dtype)
@@ -217,6 +227,18 @@ class Tensor(SimpleMathTrait):
     NOTE: A Tensor can only be scheduled once.
     """
     schedule, var_vals = create_schedule_with_vars(flatten([x.lazydata.lbs for x in (self,)+lst]))
+    # TODO: becomes_map should be returned from create_schedule_with_vars
+
+    # NOTE: this is potentially a lot of Tensors. see above about the universes
+    fixed_tensors: list[Tensor] = list(all_tensors)
+    sink = UOp.sink(*[UOp.sink(*t.lazydata.lbs) if isinstance(t.lazydata, MultiLazyBuffer) else t.lazydata for t in fixed_tensors])
+    new_sink = sink.substitute(becomes_map)
+    becomes_map.clear()
+    for t,s,ns in zip(fixed_tensors, sink.src, new_sink.src):
+      if s is ns: continue
+      if isinstance(t.lazydata, MultiLazyBuffer): t.lazydata.lbs = list(ns.src)
+      else: t.lazydata = ns
+
     return memory_planner(schedule), var_vals
 
   def schedule(self, *lst:Tensor) -> list[ScheduleItem]: