What is it about Plus that accounts for this behavior (note the middle (non)match)?
In[228]:= ClearAll[Bar];
x = Plus[Bar[1, 2], Bar[3, 4]];
In[230]:= MatchQ[x, Plus[_Bar, _]]
Out[230]= True
In[231]:= MatchQ[x, Plus[_Bar, _Bar]]
Out[231]= False
In[232]:= MatchQ[x, Plus[a_Bar, _Bar]]
Out[232]= True
I thought it might be something about the attributes:
In[233]:= Attributes[Plus]
Out[233]= {Flat, Listable, NumericFunction, OneIdentity, Orderless, Protected}
especially having read the docs about Flat/OneIdentity. But with a dummy function of equivalent attributes:
In[234]:= ClearAll[Foo];
SetAttributes[Foo, Attributes[Plus]];
x = Foo[Bar[1, 2], Bar[3, 4]];
In[237]:= MatchQ[x, Foo[_Bar, _]]
Out[237]= True
In[238]:= MatchQ[x, Foo[_Bar, _Bar]]
Out[238]= True
In[239]:= MatchQ[x, Foo[a_Bar, _Bar]]
Out[239]= True
