diff --git a/src/kyupy/logic_sim.py b/src/kyupy/logic_sim.py
index 26c361f..208ea5a 100644
--- a/src/kyupy/logic_sim.py
+++ b/src/kyupy/logic_sim.py
@@ -52,7 +52,7 @@ class LogicSim(sim.SimOps):
         """
         self.c[self.pippi_c_locs] = self.s[0, self.pippi_s_locs, :self.mdim]
 
-    def c_prop(self, sims=None, inject_cb=None):
+    def c_prop(self, sims=None, inject_cb=None, flip_line=-1):
         """Propagate the input values through the combinational circuit towards the outputs.
 
         Performs all logic operations in topological order.
@@ -68,7 +68,7 @@ class LogicSim(sim.SimOps):
         t1 = self.c_locs[self.tmp2_idx]
         if self.m == 2:
             if inject_cb is None:
-                _prop_cpu(self.ops, self.c_locs, self.c)
+                _prop_cpu(self.ops, self.c_locs, self.c, int(flip_line))
             else:
                 for op, o0l, i0l, i1l, i2l, i3l in self.ops[:,:6]:
                     o0, i0, i1, i2, i3 = [self.c_locs[x] for x in (o0l, i0l, i1l, i2l, i3l)]
@@ -298,9 +298,9 @@ class LogicSim(sim.SimOps):
 
 
 @numba.njit
-def _prop_cpu(ops, c_locs, c):
-    for op, o0, i0, i1, i2, i3 in ops[:,:6]:
-        o0, i0, i1, i2, i3 = [c_locs[x] for x in (o0, i0, i1, i2, i3)]
+def _prop_cpu(ops, c_locs, c, flip_line):
+    for op, o0l, i0l, i1l, i2l, i3l in ops[:,:6]:
+        o0, i0, i1, i2, i3 = [c_locs[x] for x in (o0l, i0l, i1l, i2l, i3l)]
         if op == sim.BUF1: c[o0]=c[i0]
         elif op == sim.INV1: c[o0] = ~c[i0]
         elif op == sim.AND2: c[o0] = c[i0] & c[i1]
@@ -335,6 +335,8 @@ def _prop_cpu(ops, c_locs, c):
         elif op == sim.OAI211: c[o0] = ~((c[i0] | c[i1]) & c[i2] & c[i3])
         elif op == sim.MUX21: c[o0] = (c[i0] & ~c[i2]) | (c[i1] & c[i2])
         else: print(f'unknown op {op}')
+        if flip_line >= 0 and o0l == flip_line:
+            c[o0] = ~c[o0]
 
 
 class LogicSim6V(sim.SimOps):