Question on the function merge_prefactor and Base.+

[peter0627ustc]

I found that the function merge_prefactor (in #58) has to confronted the different cases below and it seems a little messy, for g=propagator(𝑓⁺(1)𝑓⁻(2), factor=1):
(a)k1=(1*g+2*g)
(b)k2= linear_combination(g,g,1,2)
The structures of them are different:
(1) For k1:

(2) For k2:

I suggest these two cases should be unified to the second one in the Base.+ function.

[dcerkoney]

This is supposed to be the job of function inplace_prod. I agree that we probably want to call it directly in Base.+ and Base.* to clean up trees like k1 immediately after construction.