* src/ltlast/atomic_prop.cc, src/ltlast/automatop.cc,
src/ltlast/binop.cc, src/ltlast/bunop.cc,
src/ltlast/multop.cc, src/ltlast/unop.cc: Do not look for a key
and then insert the (key,value) on failure.  Simply insert
(key,0), and replace 0 by value on failure.  This replaces two map
lookups by one.