&templates_dir = ./templates

# This doesn't work, cannot find the generated header.txt
# I want to access the variant path for templates/ but $(TUP_VARIANTDIR) will
# follow the current Tupfile and there is no variant "anchor" like &-variables.
!add_header = | &(templates_dir)/<header_group> |> cat &(templates_dir)/header.txt %f &(templates_dir)/footer.txt > %o |> %B.gen.txt

# This works
#!add_header = | &(templates_dir)/<header_group> |> cat %f &(templates_dir)/footer.txt > %o |> %B.gen.txt