Sorted out some interesting buglets in read and print (although there is

still one I know of). More unit tests, and all pass. Not evalling yet.

Good day's work.

unit-tests/nil.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-expected=NIL
+expected=nil
 actual=`echo '()' | target/psse 2> /dev/null`
 
 if [ "${expected}" = "${actual}" ]