There are two ebuilds in the "science" overlay for more recent versions of texmacs than the ones in the normal tree. texmacs-1.0.6.6 and texmacs-1.0.6.7 are in the overlay. It looks like guile recently moved from dev-util to dev-scheme, but the two ebuilds in the science overlay are still depending on dev-util/guile >= 1.4 and are now generating errors on an "emerge -puvDN world" with the overlay enabled.
fixed in the overlay, emerges fine now.