Arch: yaourt - ERROR: One or more PGP signatures could not be verified

[rtfm_ua]

https://rtfm.co.ua/wp-content/uploads/2016/04/archlinux_logo.png Arch: yaourt - ERROR: One or more PGP signatures could not be verified
Во время установки Skype из yaourt - возникает ошибка:
$ yaourt -S skype
...
==> Verifying source file signatures with gpg...
libmng-2.0.3.tar.xz ... FAILED (unknown public key F54984BFA16C640F)
==> ERROR: One or more PGP signatures could not be verified!
==> ERROR: Makepkg was unable to build lib32-libmng

https://rtfm.co.ua/arch-yaourt-error-one-or-more-pgp-signatures-could-not-be-verified/
Arch Linux #ArchLinux