Need to handle Flat definitions correctly

[corywalker]

In[14]:= SetAttributes[foo, {Flat, Listable, NumericFunction, OneIdentity, Orderless}]

In[20]:= foo[x, y] := c

In[21]:= foo[x, y, z]

Out[21]= foo[c, z]

In[25]:= ClearAttributes[foo, {Flat, Listable, NumericFunction, OneIdentity, Orderless}]

In[28]:= ClearAll[foo]

In[31]:= foo[x, y] := c

In[32]:= foo[x, y, z]

Out[32]= foo[x, y, z]