This patch improves the behaviour of the "text()" strategy when passed
an "alphabet" which is not a strategy. The value is now interpreted
as "whitelist_characters" to "characters()" instead of a sequence for
"sampled_from()", which standardises the distribution of examples and
the shrinking behaviour.
You can get the previous behaviour by using
"lists(sampled_from(alphabet)).map("".map)" instead.
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