Refine SM-2 algorithm

[jaysonvirissimo]

Attempt to more accurately capture this SM-2 algorithm:

algorithm SM-2 is
    input:  user grade q
            repetition number n
            easiness factor EF
            interval I
    output: updated values of n, EF, and I

    if q ≥ 3 (correct response) then
        if n = 0 then
            I ← 1
        else if n = 1 then
            I ← 6
        else
            I ← round(I × EF)
        end if
        increment n
    else (incorrect response)
        n ← 0
        I ← 1
    end if
    
    EF ← EF + (0.1 − (5 − q) × (0.08 + (5 − q) × 0.02))
    if EF < 1.3 then
        EF ← 1.3
    end if
    
    return (n, EF, I)