def _letter(x: int):
    """Helper function to convert a number to a unique sequence of letters
    Uses first 10 letters for a base 10 representation"""
    first_10 = {(0): 'a', (1): 'b', (2): 'c', (3): 'd', (4): 'e', (5): 'f',
        (6): 'g', (7): 'h', (8): 'i', (9): 'j'}
    x = str(x)
    rs = ''
    for num in x:
        rs += first_10[int(num)]
    return rs
