public void MakeSubstitution(List <TokenCollection> previous, Request next)
{
// Get token from previous sequence and replace new value with old one.
IToken?replacement = source.FindClosestEquivalent(previous[sourceResponse]);
if (replacement == null)
{
throw new Exception($"Unable to find token '{source.Name}' '{source.ToString()}'.");
}
target.ReplaceValue(next, replacement.Value);
}