<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def deep_merge(source, dest):
 """ Deep merges source dict into dest dict. This code was taken directly from the mongothon project: https://github.com/gamechanger/mongothon/tree/master/mongothon """

    for key, value in source.items():
        if key in dest:
            if isinstance(value, dict) and isinstance(dest[key], dict):
                deep_merge(value, dest[key])
                continue
            elif isinstance(value, list) and isinstance(dest[key], list):
                for item in value:
                    if item not in dest[key]:
                        dest[key].append(item)
                continue
        dest[key] = value