On reading interface3.pdf (July 15, 2015) I see that spaces are not 
allowed in paths for \file_path_include:n and \file_path_remove:n. To my 
(mild) surprise I found that \file_if_exist:nTF functioned happily with 
paths that included spaces, or at least if the current *.tex document 
was in a directory at the end of a path with spaces, \file_if_exist:nTF 
was perfectly capable of finding other files in the same directory. I 
don't doubt there are underlying reasons for the different behaviours, 
but to someone not steeped in TeX lore, this feels confusing. The 
\file_path_ functions should also be indifferent to spaces in paths.

Andrew Parsloe

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