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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def is_full_mxid(user_string):
 """Returns True if a string is a valid mxid."""

    if not user_string[0] == "@":
        return False
    parts = user_string[1:].split(":")
    localpart_chars = ascii_lowercase + digits + "._-="
    if not (len(parts) == 2 and all([i in localpart_chars for i in parts[0]])):
        return False
    return True