def every_nth_character(s: str, n: int) -> str:
    """Return a string containing every nth character from s, starting at index 0.

    Precondition: n > 0

    >>> every_nth_character('Computer Science', 3)
    'CpeSee'
    """

    result = ''
    i = 0

    while ???:
        result = result + s[i]
        i = ???

    return result


def find_letter_n_times(s: str, letter: str, n: int) -> str:
    """Return the smallest substring of s starting from index 0 that contains
    n occurrences of letter.

    Precondition: letter occurs at least n times in s

    >>> find_letter_n_times('Computer Science', 'e', 2)
    'Computer Scie'
    """

    i = 0
    count = 0

    while ???:
        if ???:
            count = count + 1
        i = i + 1

    return ???


def count_collatz_steps(n: int) -> int:
    """Return the number of steps it takes to reach 1 by applying the two rules
    of the Collatz conjecture beginning from the positive integer n.

    Precondition: n >= 1

    >>> count_collatz_steps(6)
    8
    """


def ???:
    """


    """

    i = 0

    while i < len(s) and s[i] not in '0123456789':
        i = i + 1
    return i