2501: use a letter in dummy argument names of unknown functions.
2501: use a letter in dummy argument names of unknown functions.

Now it's clearer when pretty printing that the derivatives are taken
with respect to a dummy symbol, not a number:

    >>> f(g(x)).diff(x)
    d        ⎛ d        ⎞│
    ──(g(x))⋅⎜───(f(ξ₁))⎟│
    dx       ⎝dξ₁       ⎠│ξ₁=g(x)

cccfran/sympy,bukzor/sympy,jamesblunt/sympy,skidzo/sympy,ahhda/sympy,sahmed95/sympy,cswiercz/sympy,yashsharan/sympy,souravsingh/sympy,kaichogami/sympy,liangjiaxing/sympy,garvitr/sympy,Davidjohnwilson/sympy,shipci/sympy,Vishluck/sympy,farhaanbukhsh/sympy,lidavidm/sympy,postvakje/sympy,Mitchkoens/sympy,hargup/sympy,oliverlee/sympy,saurabhjn76/sympy,garvitr/sympy,shipci/sympy,Mitchkoens/sympy,jbbskinny/sympy,Designist/sympy,Designist/sympy,maniteja123/sympy,drufat/sympy,kumarkrishna/sympy,abloomston/sympy,cccfran/sympy,wyom/sympy,wyom/sympy,skidzo/sympy,cswiercz/sympy,atreyv/sympy,hrashk/sympy,madan96/sympy,cswiercz/sympy,Curious72/sympy,beni55/sympy,kumarkrishna/sympy,sampadsaha5/sympy,lidavidm/sympy,asm666/sympy,oliverlee/sympy,abloomston/sympy,vipulroxx/sympy,atreyv/sympy,drufat/sympy,wanglongqi/sympy,kevalds51/sympy,Arafatk/sympy,ChristinaZografou/sympy,jerli/sympy,chaffra/sympy,kaichogami/sympy,Gadal/sympy,Arafatk/sympy,Sumith1896/sympy,lindsayad/sympy,skidzo/sympy,MridulS/sympy,MridulS/sympy,lindsayad/sympy,chaffra/sympy,sahilshekhawat/sympy,kmacinnis/sympy,shikil/sympy,Titan-C/sympy,dqnykamp/sympy,asm666/sympy,Sumith1896/sympy,rahuldan/sympy,abhiii5459/sympy,MechCoder/sympy,hargup/sympy,aktech/sympy,MechCoder/sympy,Curious72/sympy,Titan-C/sympy,AunShiLord/sympy,jerli/sympy,diofant/diofant,pandeyadarsh/sympy,souravsingh/sympy,ga7g08/sympy,jbbskinny/sympy,pbrady/sympy,moble/sympy,grevutiu-gabriel/sympy,farhaanbukhsh/sympy,jaimahajan1997/sympy,iamutkarshtiwari/sympy,kaushik94/sympy,shikil/sympy,mcdaniel67/sympy,skirpichev/omg,flacjacket/sympy,emon10005/sympy,AkademieOlympia/sympy,minrk/sympy,asm666/sympy,Shaswat27/sympy,jaimahajan1997/sympy,shipci/sympy,AunShiLord/sympy,Gadal/sympy,srjoglekar246/sympy,pandeyadarsh/sympy,atsao72/sympy,aktech/sympy,drufat/sympy,atsao72/sympy,emon10005/sympy,dqnykamp/sympy,abhiii5459/sympy,amitjamadagni/sympy,sahmed95/sympy,kaushik94/sympy,wyom/sympy,Shaswat27/sympy,minrk/sympy,toolforger/sympy,pandeyadarsh/sympy,moble/sympy,mafiya69/sympy,Arafatk/sympy,mcdaniel67/sympy,yukoba/sympy,MechCoder/sympy,ga7g08/sympy,moble/sympy,liangjiaxing/sympy,Davidjohnwilson/sympy,kaichogami/sympy,atsao72/sympy,madan96/sympy,dqnykamp/sympy,saurabhjn76/sympy,beni55/sympy,ChristinaZografou/sympy,emon10005/sympy,wanglongqi/sympy,toolforger/sympy,jamesblunt/sympy,lidavidm/sympy,Titan-C/sympy,shikil/sympy,Davidjohnwilson/sympy,debugger22/sympy,Curious72/sympy,VaibhavAgarwalVA/sympy,kumarkrishna/sympy,meghana1995/sympy,bukzor/sympy,sahilshekhawat/sympy,postvakje/sympy,debugger22/sympy,VaibhavAgarwalVA/sympy,saurabhjn76/sympy,pbrady/sympy,toolforger/sympy,maniteja123/sympy,bukzor/sympy,Mitchkoens/sympy,Gadal/sympy,kevalds51/sympy,postvakje/sympy,grevutiu-gabriel/sympy,Sumith1896/sympy,madan96/sympy,VaibhavAgarwalVA/sympy,yashsharan/sympy,jbbskinny/sympy,meghana1995/sympy,grevutiu-gabriel/sympy,hargup/sympy,jamesblunt/sympy,atreyv/sympy,AunShiLord/sympy,ChristinaZografou/sympy,ahhda/sympy,Designist/sympy,kmacinnis/sympy,aktech/sympy,oliverlee/sympy,pbrady/sympy,ahhda/sympy,sunny94/temp,sunny94/temp,iamutkarshtiwari/sympy,yukoba/sympy,sampadsaha5/sympy,vipulroxx/sympy,hrashk/sympy,beni55/sympy,maniteja123/sympy,lindsayad/sympy,mcdaniel67/sympy,cccfran/sympy,farhaanbukhsh/sympy,sahmed95/sympy,kaushik94/sympy,sahilshekhawat/sympy,amitjamadagni/sympy,AkademieOlympia/sympy,sampadsaha5/sympy,sunny94/temp,souravsingh/sympy,garvitr/sympy,chaffra/sympy,rahuldan/sympy,yukoba/sympy,meghana1995/sympy,vipulroxx/sympy,mafiya69/sympy,Vishluck/sympy,jaimahajan1997/sympy,iamutkarshtiwari/sympy,jerli/sympy,ga7g08/sympy,liangjiaxing/sympy,debugger22/sympy,kevalds51/sympy,kmacinnis/sympy,wanglongqi/sympy,Shaswat27/sympy,abloomston/sympy,AkademieOlympia/sympy,hrashk/sympy,MridulS/sympy,Vishluck/sympy,yashsharan/sympy,mafiya69/sympy,rahuldan/sympy,abhiii5459/sympy