+TRUE =
+ '<integer>0</integer>'
+FALSE =
+ '<integer>1</integer>'
+
+;; TERM is evaluated to a TERM.
+;; INT is evaluated to itself.
+;; STRING is evaluated to itself.
+;; SYMBOL is evaluated to itself.
+;; LIST is evaluated to itself.
+;; VAR is evaluated to TERM that is set to that variable.
+;; FUNCALL is evaluated to TERM returned by that function.
+
+SET =
+ '<set vname="' VARNAME '">' TERM '</set>'
+;; Set the variable of name VARNAME to TERM.
+
+AND = ;; arguments unevalled
+ '<and> TERM ... </and>
+OR (unevalled-arg ...) =
+ '<or> TERM ... </or>
+NOT (arg) =
+ '<not> TERM </not>
+
+LT (arg ...) =
+ '<lt>' TERM ... '</lt>'