yosys-witness concat yw trace files

Intended for use with SCY to combine sequential cover traces.
Arbitrary number of inputs, will load all and attempt to join them.  Each trace will be replayed one after the other, in the same order as the files are provided.
Mismatch in init_only fields seems to work fine, with values in subsequent traces being assigned in the initial only if they weren't previously defined.
Uncertain if a mismatch in non init_only fields will cause problems.
Fixes WitnessSig.__eq__().
Adds helper functions to WitnessValues and ReadWitness classes.

backends/smt2/ywio.py

@@ -165,8 +165,8 @@ class WitnessSig:
         else:
             return f"{pretty_path(self.path)}[{self.offset}]"
 
-    def __eq__(self):
-        return self.sort_key
+    def __eq__(self, other):
+        return self.sort_key == other.sort_key
 
     def __hash__(self):
         return hash(self.sort_key)
@@ -294,6 +294,16 @@ class WitnessValues:
 
         return sorted(signals), missing_signals
 
+    def __add__(self, other: "WitnessValues"):
+        new = WitnessValues()
+        new += self
+        new += other
+        return new
+
+    def __iadd__(self, other: "WitnessValues"):
+        for key, value in other.values.items():
+            self.values.setdefault(key, value)
+        return self
 
 class WriteWitness:
     def __init__(self, f, generator):
@@ -380,13 +390,24 @@ class ReadWitness:
 
         self.bits = [step["bits"] for step in data["steps"]]
 
+    def init_step(self):
+        return self.step(0)
+    
+    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:])
+        return values
+
     def step(self, t):
         values = WitnessValues()
         values.unpack(self.sigmap, self.bits[t])
         return values
 
-    def steps(self):
-        for i in range(len(self.bits)):
+    def steps(self, start=0):
+        for i in range(start, len(self.bits)):
             yield i, self.step(i)
 
     def __len__(self):