diff --git a/backends/smt2/witness.py b/backends/smt2/witness.py
index 8e13cba27..baac3176d 100644
--- a/backends/smt2/witness.py
+++ b/backends/smt2/witness.py
@@ -88,14 +88,31 @@ If two or more inputs are provided they will be concatenated together into the o
 """)
 @click.argument("inputs", type=click.File("r"), nargs=-1)
 @click.argument("output", type=click.File("w"))
-def yw2yw(inputs, output):
+@click.option("--append", "-p", type=int, multiple=True,
+              help="Number of steps (+ve or -ve) to append to end of input trace. "
+                  +"Can be defined multiple times, following the same order as input traces. ")
+def yw2yw(inputs, output, append):
     outyw = WriteWitness(output, "yosys-witness yw2yw")
     join_inputs = len(inputs) > 1
     inyws = {}
-    for input in inputs:
+
+    if not append:
+        # default to 0
+        append = [0] * len(inputs)
+    if len(append) != len(inputs):
+        print(f"Mismatch in number of --append values ({len(append)}) and input traces ({len(inputs)}).")
+        sys.exit(1)
+
+    for (input, p) in zip(inputs, append):
         if (join_inputs):
             click.echo(f"Loading signals from yosys witness trace {input.name!r}...")
         inyw = ReadWitness(input)
+        if p:
+            click.echo(f"  appending {p} steps")
+            if (p + len(inyw) <= 0):
+                click.echo(f"  skipping {input.name!r} (only {len(inyw)} steps to skip)")
+                continue
+            inyw.append_steps(p)
         inyws[input] = inyw
         for clock in inyw.clocks:
             if clock not in outyw.clocks:
@@ -119,7 +136,7 @@ def yw2yw(inputs, output):
         for t, values in inyw.steps(1):
             outyw.step(values)
 
-        click.echo(f"Copied {t + 1} time steps.")
+        click.echo(f"  copied {t + 1} time steps.")
         first_witness = False
 
     outyw.end_trace()
diff --git a/backends/smt2/ywio.py b/backends/smt2/ywio.py
index 2b897200f..4e95f8c33 100644
--- a/backends/smt2/ywio.py
+++ b/backends/smt2/ywio.py
@@ -393,12 +393,16 @@ class ReadWitness:
     def init_step(self):
         return self.step(0)
     
+    def non_init_bits(self):
+        if len(self) > 1:
+            return len(self.bits[1])
+        else:
+            return sum([sig.width for sig in self.signals if not sig.init_only])
+    
     def first_step(self):
         values = WitnessValues()
-        if len(self.bits) <= 1:
-            raise NotImplementedError("ReadWitness.first_step() not supported for less than 2 steps")
-        non_init_bits = len(self.bits[1])
-        values.unpack(WitnessSigMap([sig for sig in self.signals if not sig.init_only]), self.bits[0][-non_init_bits:])
+        # may have issues when non_init_bits is 0
+        values.unpack(WitnessSigMap([sig for sig in self.signals if not sig.init_only]), self.bits[0][-self.non_init_bits():])
         return values
 
     def step(self, t):
@@ -410,5 +414,13 @@ class ReadWitness:
         for i in range(start, len(self.bits)):
             yield i, self.step(i)
 
+    def append_steps(self, t):
+        if not t:
+            pass
+        elif t < 0:
+            self.bits = self.bits[:t]
+        else:
+            self.bits.extend(["0"*self.non_init_bits()]*t)
+
     def __len__(self):
         return len(self.bits)