Sets the write stream to the file named
The write stream is used by PRINT, SHOW, etc.
The file must already be open with OPENWRITE, OPENAPPEND, or OPENUPDATE.
If the input is the empty list, then the write stream becomes the Commander, as usual.
Changing the write stream does not close the file that was previously the write stream, so it is possible to alternate between files.
OPENWRITE "dummy.fil SETWRITE "dummy.fil PRINT "Hello PRINT [Good Bye] SETWRITE  CLOSE "dummy.fil OPENREAD "dummy.fil SETREAD "dummy.fil REPEAT 2 [SHOW READLIST][Hello] [Good Bye]
SETREAD  CLOSE "dummy.fil