tidyr mutate equivalent without need for scoping
R equivalent of
mutate(df, c=a-b) 
in pandas is this: 
df.assign(c=df['a']-df['b'])

I wish it was like this: 

df.assign(c=a-b)

In more complex examples,
df.assign(c=a-b, d=c**2, f=np.sin(c) )
 it would make it easier to focus on math . It think implementation could be similar to df.query(...) syntax. 


