<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def parse_template(input_filename, output_filename=''):
 """ Parses a template file Replaces all occurences of @@problem_id@@ by the value of the 'problem_id' key in data dictionary input_filename: file to parse output_filename: if not specified, overwrite input file """

    data = load_input()
    with open(input_filename, 'rb') as file:
        template = file.read().decode("utf-8")
    
    # Check if 'input' in data
    if not 'input' in data:
        raise ValueError("Could not find 'input' in data")
    
    # Parse template
    for field in data['input']:
        subs = ["filename", "value"] if isinstance(data['input'][field], dict) and "filename" in data['input'][field] and "value" in data['input'][field] else [""]
        for sub in subs:
            displayed_field = field + (":" if sub else "") + sub
            regex = re.compile("@([^@]*)@" + displayed_field + '@([^@]*)@')
            for prefix, postfix in set(regex.findall(template)):
                if sub == "value":
                    text = open(data['input'][field][sub], 'rb').read().decode('utf-8')
                elif sub:
                    text = data['input'][field][sub]
                else:
                    text = data['input'][field]
                rep = "\n".join([prefix + v + postfix for v in text.splitlines()])
                template = template.replace("@{0}@{1}@{2}@".format(prefix, displayed_field, postfix), rep)
    
    if output_filename == '':
        output_filename=input_filename
    
    # Ensure directory of resulting file exists
    try:
        os.makedirs(os.path.dirname(output_filename))
    except OSError as e:
        pass

    # Write file
    with open(output_filename, 'wb') as file:
        file.write(template.encode("utf-8"))