Tex: Problem with command expansion in xstring macro? "Incomplete iffalse" 

Problem with command expansion in xstring macro? "Incomplete iffalse"
I hope you found a solution that worked for you :)
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Thanks to all those great people for their contributions!
(tex.stackexchange.com/users/312438/tired63194)tired63194
(tex.stackexchange.com/users/4427/egreg)egreg
(tex.stackexchange.com/users/51799/wipet)wipet
(tex.stackexchange.com/users/25858/steven-b-segletes)Steven B. Segletes
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If anything is off, please write me at peter D.O.T schneider A.T ois42.de
fragile macros expansion