Start some internal functions with '_'
Start some internal functions with '_'

The functions are _solve_variation_of_parameters() and
_solve_undetermined_coefficients.  This is for example
so they won't be caught by the doctest coverage test.

Signed-off-by: Ondrej Certik <b816faa87b7d35274d2e545c5be11ed4376f3ccf@certik.cz>

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