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fail
fail, keyword
The keyword fail
prevents Envision from replacing an empty selection by a default value.
It applies to lookups:
table T[x] = with
[| "aa" as X, 11 as Y|]
[| "bb", 12 |]
[| "cc", 13 |]
a = T.Y["bb"] default fail
show scalar "" with a // displays 12