def add(self, key, val):
        key_lower = key.lower()
        new_vals = key, val
        # Keep the common case aka no item present as fast as possible
        vals = self._container.setdefault(key_lower, new_vals)
        if new_vals is not vals:
            # new_vals was not inserted, as there was a previous one
            if isinstance(vals, list):
                # If already several items got inserted, we have a list
                vals.append(val)
            else:
                # vals should be a tuple then, i.e. only one item so far
                # Need to convert the tuple to list for further extension
                self._container[key_lower] = [vals[0], vals[1], val]
Adds a (name, value) pair, doesn't overwrite the value if it already
dataset/ETHPy150Open sigmavirus24/betamax/betamax/headers.py/HTTPHeaderDict.add