L= sum<j=0..N-1>( r^j )
    L= 1+r+..+r^(N-1)
    r*L - L = r*( 1+r+..+r^(N-2)+r^(N-1) ) - ( 1+r+..+r^(N-1) )
    r*L - L = ( r+r^2+..+r^(N-1)+r^N ) - ( 1+r+..+r^(N-1) )
    r*L - L = r+r^2+..+r^(N-1) + r^N - ( 1 + r+..+r^(N-1) )
    r*L - L = r+r^2+..+r^(N-1) + r^N - 1 - ( r+..+r^(N-1) )