According to the "IPA Ruby Standardization WG Draft August 25, 2010"

(page 97)
11.5.2.5 The begin expression
  Semantics
  c.2
"If the exception-class-list is omitted in the rescue-clause, and if E
is an instance of the class StandardError, the rescue-clause handles E."

If any Ruby interpreter handles any exception that is not a sub-class of
StandardError in a rescue-clause (no exception-class-list), then it is
in error.

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