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A result object which either is in success state containing a
non-null boolean value, or in error state containing a
non-null error value.
A result object which either is in success state, where it may contain a
non-null success value or be empty, or it may be in error state
containing a non-null error value.
A result object which either is in success state containing a
non-null success value, or in error state containing a
non-null error value.
A result object which either is in success state with no value, or in error
state containing a non-null error value.