Chagrin

 It is important to recognize that in the hump formula for Bros,

one is inside the update function. Anything we assign will happen

in dt. If we merely add velocity to position, Mario will fly off

the page. It will do this more slowly if we add *dt to velocity.

Delta is interesting because it puts things in the player's control.

Mario moves as a result of player input. Delta itself is 'normalizeInplace()'.

ie it will reset to 1 after every use, just as dt reverts to 0. And

 acceleration - to Newton's chagrin - is just a booster to delta in this game...