The Operator "String" is not defined

This error occurs on the client machine (not in the debugger) when there is no way to cast the result as a string, so it can't be represented. For instance:

set of (1;2;3)

This expression refers to a legitimate set, but the Relevance language doesn't know how to represent it in the debugger. To see the set, use a syntax like this:

Q: elements of set of (1;2;3)
A: 1
A: 2
A: 3

Note, however, that although the client gets a string error, the Fixlet debugger will produce a different error:

Q: set of (1;2;3)
E: This expression evaluates to an unrepresentable object of type "integer set"