Use set().union instead of set.union
Use set().union instead of set.union

This makes it work with no arguments. Thanks to @kjordahl for pointing this
out.

jaimahajan1997/sympy,skirpichev/omg,Designist/sympy,Gadal/sympy,mcdaniel67/sympy,souravsingh/sympy,VaibhavAgarwalVA/sympy,farhaanbukhsh/sympy,debugger22/sympy,aktech/sympy,madan96/sympy,sahmed95/sympy,emon10005/sympy,vipulroxx/sympy,aktech/sympy,cswiercz/sympy,hargup/sympy,hargup/sympy,bukzor/sympy,moble/sympy,grevutiu-gabriel/sympy,hargup/sympy,jaimahajan1997/sympy,skidzo/sympy,Davidjohnwilson/sympy,garvitr/sympy,AunShiLord/sympy,Mitchkoens/sympy,rahuldan/sympy,Sumith1896/sympy,toolforger/sympy,AkademieOlympia/sympy,abhiii5459/sympy,atreyv/sympy,Arafatk/sympy,ga7g08/sympy,Arafatk/sympy,yukoba/sympy,rahuldan/sympy,liangjiaxing/sympy,kumarkrishna/sympy,mcdaniel67/sympy,jamesblunt/sympy,sampadsaha5/sympy,maniteja123/sympy,jamesblunt/sympy,wyom/sympy,drufat/sympy,pandeyadarsh/sympy,jerli/sympy,pbrady/sympy,yashsharan/sympy,oliverlee/sympy,mcdaniel67/sympy,emon10005/sympy,ChristinaZografou/sympy,vipulroxx/sympy,MechCoder/sympy,ahhda/sympy,chaffra/sympy,saurabhjn76/sympy,moble/sympy,lindsayad/sympy,madan96/sympy,Gadal/sympy,jerli/sympy,Curious72/sympy,bukzor/sympy,kumarkrishna/sympy,jerli/sympy,kaushik94/sympy,kaichogami/sympy,drufat/sympy,pbrady/sympy,shikil/sympy,grevutiu-gabriel/sympy,yashsharan/sympy,vipulroxx/sympy,debugger22/sympy,mafiya69/sympy,pandeyadarsh/sympy,sampadsaha5/sympy,farhaanbukhsh/sympy,AkademieOlympia/sympy,atreyv/sympy,kevalds51/sympy,jbbskinny/sympy,MechCoder/sympy,maniteja123/sympy,diofant/diofant,pandeyadarsh/sympy,Designist/sympy,asm666/sympy,Vishluck/sympy,Titan-C/sympy,sahilshekhawat/sympy,liangjiaxing/sympy,garvitr/sympy,postvakje/sympy,kumarkrishna/sympy,souravsingh/sympy,kevalds51/sympy,iamutkarshtiwari/sympy,Shaswat27/sympy,atsao72/sympy,ahhda/sympy,abhiii5459/sympy,mafiya69/sympy,beni55/sympy,postvakje/sympy,Sumith1896/sympy,ChristinaZografou/sympy,beni55/sympy,wanglongqi/sympy,MechCoder/sympy,Mitchkoens/sympy,emon10005/sympy,cswiercz/sympy,kevalds51/sympy,debugger22/sympy,aktech/sympy,jaimahajan1997/sympy,ga7g08/sympy,mafiya69/sympy,atsao72/sympy,Vishluck/sympy,wanglongqi/sympy,liangjiaxing/sympy,abhiii5459/sympy,drufat/sympy,maniteja123/sympy,wanglongqi/sympy,atreyv/sympy,jamesblunt/sympy,jbbskinny/sympy,Davidjohnwilson/sympy,VaibhavAgarwalVA/sympy,skidzo/sympy,yashsharan/sympy,Shaswat27/sympy,pbrady/sympy,moble/sympy,jbbskinny/sympy,wyom/sympy,madan96/sympy,rahuldan/sympy,beni55/sympy,kaichogami/sympy,asm666/sympy,chaffra/sympy,iamutkarshtiwari/sympy,kaichogami/sympy,kaushik94/sympy,yukoba/sympy,saurabhjn76/sympy,Davidjohnwilson/sympy,meghana1995/sympy,Vishluck/sympy,sahilshekhawat/sympy,Titan-C/sympy,Mitchkoens/sympy,AunShiLord/sympy,asm666/sympy,AkademieOlympia/sympy,Shaswat27/sympy,skidzo/sympy,meghana1995/sympy,yukoba/sympy,Arafatk/sympy,cswiercz/sympy,chaffra/sympy,shikil/sympy,saurabhjn76/sympy,sahmed95/sympy,toolforger/sympy,Gadal/sympy,farhaanbukhsh/sympy,shikil/sympy,ChristinaZografou/sympy,garvitr/sympy,bukzor/sympy,abloomston/sympy,lindsayad/sympy,iamutkarshtiwari/sympy,oliverlee/sympy,Sumith1896/sympy,Titan-C/sympy,AunShiLord/sympy,ahhda/sympy,Curious72/sympy,VaibhavAgarwalVA/sympy,postvakje/sympy,lindsayad/sympy,sahilshekhawat/sympy,Designist/sympy,sampadsaha5/sympy,abloomston/sympy,souravsingh/sympy,meghana1995/sympy,abloomston/sympy,kaushik94/sympy,oliverlee/sympy,Curious72/sympy,ga7g08/sympy,wyom/sympy,grevutiu-gabriel/sympy,sahmed95/sympy,atsao72/sympy,toolforger/sympy