Thanks!
I think, it's a bit confusing, that the dafault value is accessed by
"hash["a"]", i.e. the term for accessing a value according to key "a".
The constuctor "Hash.new {block}" works for me:
ahash = Hash.new {|hash,key| hash[key] = ""}
=> {}
ahash["a"] << "vier"
=> "vier"
ahash
=> {"a"=>"vier"}
Best regards
Ralf