randomly spawn an advanced enemy
use auto to remove local typedef
predicates must be copy constructable, KISS
extract bounce_of_the_bounds_x
bounce only normal enemies
bounce correctly advanced enemies w/o increasing y position
spawn advanced enemy only when there are no ther enemies on the 0 line
move faster
score normal & advanced enemies separately
avoid double booking
advanced enemy has it's own reload time
moar braces
add fire_rate, not it is configurable per unit
add kAdvancedEnemySpawnRate
advanced enemy should have it's own fire rate
const paranoia
simplify unit_pick_randomly, don't requre + operator
adaptors magic, simplify firing
more adaptors magic, gather all enemies at the bottom
fix types