I've just found the syntax <<: [*foo, *bar] as a workaround. If that's actually the only way - as spec - to define multiple merge, please feel free to close. Thank you!
Ruamel.yaml comsiders two merge keys as a duplicate key
Added sample psql session file
Added PostgreSQL specific SQL and interactive session lexers
PostgreSQL keywords updated to version 9.0
Lexers mapping updated to contain PostgreSQL's
Multi-character operators in PostgreSQL
Added prompt style => in psql lexer