argument in open and
defaults to the value of
, checks are made to ensure that the resulting stream's stream-element-type is compatible with its external format:
:io, the element-type argument must be a supertype of the type of characters produced by the external format.
:io, the element-type argument must be a subtype of the type of characters accepted by the external format
If the element-type argument doesn't satisfy these requirements, an error is signalled.
the system chooses the stream-element-type on the basis of the external format.