"If port is an output port, set-port-position! first flushes port."

Should probably add: "The port is flushed even when the new port
position is the same as the old one." It's simpler to flush even in that
case, since people may come to depend on the flushing side effect, and a
piece of code may get the new position from a variable that may or may
not be equal to the old position.