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ReplaceAll feature


In the following, why is Out[1] different from Out[2] and Out[3]:

Mathematica 2.0 for SGI Iris
Copyright 1988-91 Wolfram Research, Inc.
 -- Terminal graphics initialized --

In[1]:= (a+b+c+d)/.{a+b->x,c+d->y}

Out[1]= c + d + x

In[2]:= (a+b+c+d)/.a+b->x/.c+d->y

Out[2]= x + y

In[3]:= (a+b+c+d)//.{a+b->x,c+d->y}

Out[3]= x + y

I guess it has something to do with Flat. This is not a big problem, but IF
it can be easily fixed, it should be fixed. But maybe there are some "deep"
reasons why it behaves like this ?
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