Stream pattern components are matched against the corresponding elements of a stream. Finally, the component ident simply binds the identifier to the stream being matched. If the following components do not match, the exception Stream. Error is raised. If none of the first components of the stream patterns match, the exception Stream.
Failure is raised.
Failure exception causes the next alternative to be tried, if it occurs during the matching of the first element of a stream, before matching has committed to one pattern. The streams hold the count of their elements discarded. The optional pattern before the first stream pattern is bound to the stream count before the matching. The exception Stream. This expression is evaluated only in case of error.