They don't have to agree on anything, just use readline. It's already used by, or optionally used by, tons of software. Here's a non-exhaustive list that immed

They don't have to agree on anything, just use readline. It's already used by, or optionally used by, tons of software. Here's a non-exhaustive list that immediately come to mind: bash rc psql bc ftp GDB GPG Guile Lua NetworkManager Parted Python Ruby SQLite wpa_supplicant