Sets the read stream to the file named
The read stream is used by READLIST, READWORD, etc.
The file must already be open with OPENREAD or OPENUPDATE.
If the input is the empty list, then the read stream becomes the Commander, as usual.
Changing the read stream does not close the file that was previously the read 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