randltl: fix generation without unary operators

* spot/tl/randomltl.hh (has_unary_ops): New method.
* spot/tl/randomltl.cc: Avoid creating subformulas of even size
when we do not have unary operators.
* tests/core/randpsl.test: Test it.
* NEWS: Mention it.

tests/core/randpsl.test

@@ -36,3 +36,21 @@ test `wc -l < formulas` = 50
 
 
 randltl --psl --sere-priorities=first_match=10 -n 100 2 | grep first_match
+
+# the random generator had trouble generating formulas of the proper size when
+# unary operators were disabled
+P=true=0,false=0,not=0
+randltl --tree-size=19 -B --boolean-prio=$P 1000 -n10 --stats=%a >out
+cat >expected <<EOF
+10
+10
+10
+10
+10
+10
+10
+10
+10
+10
+EOF
+diff expected out