Consider the following dask

In [2]: def inc(x): return x + 1

In [3]: d = {'x': 1, 'y': (inc, 'x'), 'z': (inc, 'x'), 'out': (add, 'y', 10)}

If we go ahead and get out the out key

>>> from dask.threaded import get
>>> get(d, 'out')

We end up doing excess work computing 'z'.

It would be nice to have a function that takes a dask and a key or list of keys and removes all tasks not necessary to compute those keys.

>>> cull(d, ['out'])
{'x': 1, 'y': (inc, 'x'), 'out': (add, 'y', 10)}