def mark_char(text, idx):
    """
    Prominently marks the *idx* char in *text*.

    >>> mark_char('hello world', 0)
    '▸▸▸h◂◂◂ello world'
    >>> mark_char('hello world', 2)
    'he▸▸▸l◂◂◂lo world'
    >>> mark_char('hello world', 10)
    'hello worl▸▸▸d◂◂◂'

    >>> mark_char('hello world', 11)
    Traceback (most recent call last):
        ...
    IndexError: string index out of range

    >>> mark_char('', 0)
    Traceback (most recent call last):
        ...
    IndexError: string index out of range
    """
    return text[0:idx] + '▸▸▸' + text[idx] + '◂◂◂' + text[idx+1:]