> |if k in d| is much more efficient than |if k in d.keys()| because it 
> will not running a method and allocate memory for the return of that 
> method.
>
And also because d.keys() is a list, so 'if k in d.keys()' is linear 
time (it has to search the unordered list), while 'if k in d' is 
near-constant time (just checks the hash code).