Arrange multops so that Boolean arguments come first.

This helps recursive implication checks.  Also order
atomic propositions with strverscmp().

* src/ltlast/formula.hh (formula_ptr_less_than_multop,
is_literal, atomic_prop_cmp): New.
* src/ltlast/formula.cc (is_literal, atomic_prop_cmp): Implement them.
* src/ltlast/multop.cc: Use formula_ptr_less_than_multop.
* src/ltltest/isop.test, src/ltltest/ltlfilt.test,
src/tgbatest/det.test, src/tgbatest/dstar.test,
src/tgbatest/explicit.test, src/tgbatest/explpro2.test,
src/tgbatest/explpro3.test, src/tgbatest/explprod.test,
src/tgbatest/nondet.test, src/tgbatest/tripprod.test: Adjust tests.
* NEWS: Mention the new order.

src/ltltest/isop.test

@@ -33,9 +33,9 @@ run 0 ../../bin/ltlfilt --boolean-to-isop input > output
 
 cat> expected<<EOF
 (!a & !b) | (b & d)
-(b | !a) & (d | !b) & Xc
+(!a | b) & (!b | d) & Xc
 GF(b | (a & d))
-{{{{!a && !b} | {b && d}}}[*];a[*]}<>-> ((a & !b) | (b & !a))
+{{{{!a && !b} | {b && d}}}[*];a[*]}<>-> ((!a & b) | (a & !b))
 EOF
 
 cat output
@@ -46,9 +46,9 @@ run 0 ../../bin/ltlfilt input > output
 
 cat> expected<<EOF
 (a -> b) & (b -> d)
-(a -> b) & Xc & (b -> d)
+(a -> b) & (b -> d) & Xc
 GF((a | b) & (b | d))
-{{{{a -> b} && {b -> d}}}[*];a[*]}<>-> ((a | b) & (!b | !a))
+{{{{a -> b} && {b -> d}}}[*];a[*]}<>-> ((a | b) & (!a | !b))
 EOF
 
 cat output

src/ltltest/ltlfilt.test

@@ -90,7 +90,7 @@ b W GFa
 Fb
 G(a & Xb)
 Xa
-F(a & X(b & !a))
+F(a & X(!a & b))
 a & (b | c)
 EOF
 
@@ -98,7 +98,7 @@ checkopt --simplify --eventual --unique <<EOF
 F(GFa | Gb)
 F(b W GFa)
 Fb
-F(a & X(b & !a))
+F(a & X(!a & b))
 EOF
 
 checkopt --safety <<EOF