The type of a sequence of messages.
f is defined at
t apply it and return the resulting sequence
Check that the entity
e is used legally and return appropriate
messages if not.
Recursively collect all messages in the given tree using the partial
messages at all nodes where it is defined.
cond is true make a singleton message list that associates the
label with the start position recorded for
value (if any).
Return a value representing no messages.