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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 show_letter(letter, text_color=None, back_color=None):
    '''
    Displays a single letter on the LED matrix.

    letter
        The letter to display
    text_color
        The color in which the letter is shown. Defaults to '[255, 255, 255]' (white).
    back_color
        The background color of the display. Defaults to '[0, 0, 0]' (black).

    CLI Example:

    .. code-block:: bash

        salt 'raspberry' sensehat.show_letter O
        salt 'raspberry' sensehat.show_letter X '[255, 0, 0]'
        salt 'raspberry' sensehat.show_letter B '[0, 0, 255]' '[255, 255, 0]'
    '''
    text_color = text_color or [255, 255, 255]
    back_color = back_color or [0, 0, 0]

    _sensehat.show_letter(letter, text_color, back_color)
    return {'letter': letter}