ABC_LOWER = "abcdefghijklmnopqrstuvwxyz"
ABC_UPPER = ABC_LOWER.upper()

def rot13(phrase):
    if not isinstance(phrase, str):
        return "Error: input is not a string!"
    out_phrase = ''
    for char in phrase:
        if char.isupper():
            out_phrase += ABC_UPPER[(ABC_UPPER.find(char)+13)%26]
        else:
            out_phrase += ABC_LOWER[(ABC_LOWER.find(char)+13)%26]
    return out_phrase
ABC_LOWER = "abcdefghijklmnopqrstuvwxyz"
ABC_UPPER = ABC_LOWER.upper()

def rot13(phrase):
    if not isinstance(phrase, str):
        return "Error: input is not a string!"
    out_phrase = ''
    for char in phrase:
        if char.isupper():
            out_phrase += ABC_UPPER[(ABC_UPPER.find(char)+13)%26]
        else:
            out_phrase += ABC_LOWER[(ABC_LOWER.find(char)+13)%26]
    return out_phrase