These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 0.97 s (backtrack: 0/20).

[ebuild  N     ] dev-lang/mlton-20180207:0/20180207::gentoo  USE="-binary -bootstrap-smlnj -doc -pax-kernel -stage3" 42751 KiB
[ebuild  N     ] sci-mathematics/twelf-1.7.1-r2:0/1.7.1::gentoo  USE="-doc -emacs -examples" 1121 KiB

Total: 2 packages (2 new), Size of downloads: 43872 KiB