Xjyuk

I'm using the chemfig package for some chemical reactions and I need to draw an arrow like the one below:

It's essentially arrow-U> but with the curved arrow positioned below the straight one. I found a solution in this previous post which works perfectly when I try it on Overleaf, but it does not work on my own TeX installation (TeXLive 2019, Windows 10). Every time I try it I get several "undefined control sequence" errors.

Any idea on why this isn't working and how to fix it? Barring that, is there any other way I can get the kind of arrow I want?

Here's an MWE (copied from the link above):

documentclassarticle
usepackagechemfig

makeatletter
definearrow5-u>%
 CF@arrow@shift@nodes#3%
 expandafterdrawexpandafter[CF@arrow@current@style](CF@arrow@start@node)--(CF@arrow@end@node)node[midway](uarrow@arctangent);%
 CF@ifempty#4
 defCF@uarrow@radius0.333
 defCF@uarrow@radius#4%
 CF@ifempty#5%
 defCF@uarrow@absangle60
 pgfmathsetmacroCF@uarrow@absangleabs(#5)
 %
 edefCF@tmp@str[CF@ifempty#1draw=noneunexpandedexpandafterCF@arrow@current@style,-]%
 expandafterdrawCF@tmp@str (uarrow@arctangent)%
 arc[radius=CF@compound@sep*CF@current@arrow@length*CF@uarrow@radius,start angle=CF@arrow@current@angle+90,delta angle=CF@uarrow@absangle]node(uarrow@start);
 %
 edefCF@tmp@str[CF@ifempty#2draw=noneunexpandedexpandafterCF@arrow@current@style,-CF]%
 expandafterdrawCF@tmp@str (uarrow@arctangent)%
 arc[radius=CF@compound@sep*CF@current@arrow@length*CF@uarrow@radius,%
 start angle=CF@arrow@current@angle+90,%
 delta angle=-CF@uarrow@absangle]%
 node(uarrow@end);
 pgfmathsetmacroCF@tmp@strCF@uarrow@radius*cos(CF@arrow@current@angle)<0?"+":"-"%
 ifdimCF@uarrow@radius pt>z@
 CF@arrow@display@label#10CF@tmp@struarrow@start#21CF@tmp@struarrow@end%
 else
 CF@arrow@display@label#20CF@tmp@struarrow@start#11CF@tmp@struarrow@end%
 fi

makeatother

begindocument
 begincenter
% schemedebugtrue % Activate if you want to see the nodes etc.
 schemestart
 arrow-u>[$x$][$y$][][.5][]
 schemestop
 endcenter
enddocument

If you look into the chemfig manual, then you will find on in section 1.3 that the private char has changed from @ to _, with more details specified in section 10. Following the instructions there, one gets

documentclassarticle
usepackagechemfig

makeatlettercatcode`_=11
definearrow5-u>%
 CF_arrowshiftnodes#3%
 expandafterdrawexpandafter[CF_arrowcurrentstyle](CF_arrowstartnode)
 --(CF_arrowendnode)node[midway](uarrow@arctangent);%
 CF_ifempty#4
 defCF_uarrowradius0.333
 defCF_uarrowradius#4%
 CF_ifempty#5%
 defCF_uarrowabsangle60
 pgfmathsetmacroCF_uarrowabsangleabs(#5)
 %
 edefCF_tmpstr[CF_ifempty#1draw=noneunexpandedexpandafterCF_arrowcurrentstyle,-]%
 expandafterdrawCF_tmpstr (uarrow@arctangent)%
 arc[radius=CF_compoundsep*CF_currentarrowlength*CF_uarrowradius,start angle=CF_arrowcurrentangle+90,delta angle=CF_uarrowabsangle]node(uarrowstart);
 %
 edefCF_tmpstr[CF_ifempty#2draw=noneunexpandedexpandafterCF_arrowcurrentstyle,-CF]%
 expandafterdrawCF_tmpstr (uarrow@arctangent)%
 arc[radius=CF_compoundsep*CF_currentarrowlength*CF_uarrowradius,%
 start angle=CF_arrowcurrentangle+90,%
 delta angle=-CF_uarrowabsangle]%
 node(uarrowend);
 pgfmathsetmacroCF_tmpstrCF_uarrowradius*cos(CF_arrowcurrentangle)<0?"+":"-"%
 ifdimCF_uarrowradius pt>z@
 CF_arrowdisplaylabel#10CF_tmpstruarrowstart#21CF_tmpstruarrowend%
 else
 CF_arrowdisplaylabel#20CF_tmpstruarrowstart#11CF_tmpstruarrowend%
 fi

catcode`_=8
makeatother

begindocument
 begincenter
% schemedebugtrue % Activate if you want to see the nodes etc.
 schemestart
 arrow-u>[$x$][$y$][][.5][]
 schemestop
 endcenter
enddocument