implement `unset` semantics as required by POSIX
diff --git a/shell/hush_test/hush-vars/unset.tests b/shell/hush_test/hush-vars/unset.tests
new file mode 100755
index 0000000..f59ce59
--- /dev/null
+++ b/shell/hush_test/hush-vars/unset.tests
@@ -0,0 +1,36 @@
+# check invalid options are rejected
+unset -
+echo $?
+unset -m a b c
+echo $?
+
+# check funky usage
+unset
+echo $?
+
+# check normal usage
+echo ___
+f=f g=g
+echo $? $f $g
+unset f
+echo $? $f $g
+unset g
+echo $? $f $g
+
+echo ___
+f=f g=g
+echo $? $f $g
+unset f g
+echo $? $f $g
+f=f g=g
+echo $? $f $g
+unset -v f g
+echo $? $f $g
+
+# check read only vars
+echo ___
+f=f g=g
+unset HUSH_VERSION
+echo $? $f $g
+unset f HUSH_VERSION g
+echo $? $f $g