I'm looking for a kind of 'urn' object but for symbols. 'urn' only works with numbers. Actually I want to use it with 'msgfile'. Does this kind of object exist ?

have a look in list-abs, i'm sure there must be a randomise list abstraction.

also, to get random output from msgfile,

[random n]
|
[goto $1, bang(
|
[msgfile]