sympy/sympy — pull request #1869 by bgee (2013-03-05 06:06:11+00:00)
Title: KroneckerDelta not canonical (issue 3479 on google code) 

Tried to use minlex to swap parameters within KroneckerDelta but preferred_index will return the first input if both parameters contain same information about fermi level. So I override a ____eq____ method(markdown keeps messing around with the underscore) .