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TagSetDelayed and UpValues


The first example below is from The Mathematica Book, and the second is from
TMGB-P.  Notice that g has an upvalue associated with it, whereas func
still has the TagSetDelayed associated with it.  Why is that?  How are
these values treated (differently) during evaluation?  

I'm confident that the upvalue for g will tried when g appears in an
argument list, and if the head of the expression inwhich it appears is
Plus, the rule will be applied.  I'm not sure what will happen with func.

In[1]:= g/:g[x_]+g[y_]:=gplus[x,y] 

In[2]:= ?g
Global`g

g[x_] + g[y_] ^:= gplus[x, y]

In[3]:= func /: D[func[x_], {x_, n_}] := derivOfFunc[x, n]

In[4]:= ?func
Global`func

func /: D[func[x_], {x_, n_}] := derivOfFunc[x, n]

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