Better Grammar#to_s, Grammar::Rule#to_s
Don't merge if no common substring found
Fix grammar#merge logic
Don't lookup empty meanings
Send proper Meaning instance to #lookup
Grammar::Rule#embed! should do local replace
Grammar::Rule's index should be single-digit
Do not learn an empty rule
Do not merge an empty rule
Fix return value of Player#lookup
grammar.rb, player.rb require meanings.rb