&root = .

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

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