'Initialise with the existing stack and the new stack.'

def __init__(self, existing_stack, new_stack, previous_stack, rollback=False):

    self.existing_stack = existing_stack
    self.new_stack = new_stack
    self.previous_stack = previous_stack
    self.rollback = rollback
    self.existing_snippets = dict(((n, r.frozen_definition()) for (n, r) in self.existing_stack.items()))
