Lior,
You want the notation package. Use the notation palette to make creating your notation easier.
Regards
Neil
Also, Look at Format]. See the Documentation [on output formats and Format here
You can define a function and have it display as your special character or you can define a variable to display as a special character.
In[1]:= hugo := Overscript["\[LeftArrow]", "+"] In[2]:= hugo Out[2]= \!\(\*OverscriptBox["\<\"\[LeftArrow]\"\>", "\<\"+\"\>"]\)
Excellent, this is exactly it.
Many thanks, Neil!
