def add(key=None, value=None, *, myst, getpass, **kwargs):
    """
    Add a new key-value pair. Accepts a optional key and an optional value for the key. If the key or value is not entered, a secure input will be prompted to enter them.
    """
    if not myst.mutable:
        return 'the myst is in read-only mode, use the enable_write command to enable editing'
    if key is None:
        key = getpass('enter the key\n')
    if key in myst:
        return 'key already exists, use update or set_pair to change existing pairs'
    if value is None:
        value = getpass(f'enter the value\n')

    myst[key] = value
    return 'pair added'