From Wolfram MonomialList[] Documentation
In[]:=
MonomialList[ax^2+bxy+cy^2,{x,y}]
Out[]=
{a
2
x
,bxy,c
2
y
}
In[]:=
Apply[Plus,%]
Out[]=
a
2
x
+bxy+c
2
y
Change the variable order, MonomialList[] works but all is lost in Plus[]
In[]:=
MonomialList[ax^2+bxy+cy^2,{y,x}]
Out[]=
{c
2
y
,bxy,a
2
x
}
In[]:=
Apply[Plus,%]
Out[]=
a
2
x
+bxy+c
2
y