Benchs: Makefile corrected for reach files

benchs/Makefile

 GO2PINS=../go2pins
 
 NBTHREADS?=1
+REACHTHREADS?=1
 OUTPUT=benchs-${NBTHREADS}.csv
 OUTPUT_BB=benchs-bb-${NBTHREADS}.csv
-OUTPUT_REACH=benchs-reach-${NBTHREADS}.csv
+OUTPUT_REACH=benchs-reach-${REACHTHREADS}.csv
 OUTPUT_LTL=benchs-ltl-${NBTHREADS}.csv
 
 LTLFILES=RERS/2016-Problem1-ltl.go RERS/2016-Problem2-ltl.go RERS/2016-Problem4-ltl.go RERS/2016-Problem5-ltl.go RERS/2016-Problem7-ltl.go RERS/2017-Problem1-ltl.go RERS/2017-Problem2-ltl.go RERS/2017-Problem4-ltl.go RERS/2017-Problem5-ltl.go RERS/2017-Problem7-ltl.go RERS/2018-Problem1-ltl.go RERS/2018-Problem2-ltl.go RERS/2018-Problem4-ltl.go RERS/2018-Problem5-ltl.go RERS/2018-Problem7-ltl.go RERS/2019-Problem1-ltl.go RERS/2019-Problem2-ltl.go RERS/2019-Problem4-ltl.go RERS/2019-Problem5-ltl.go RERS/2019-Problem7-ltl.go RERS/2020-Problem1-ltl.go RERS/2020-Problem2-ltl.go RERS/2020-Problem4-ltl.go RERS/2020-Problem5-ltl.go RERS/2020-Problem7-ltl.go 
@@ -43,11 +44,13 @@ ${LTLOUTPUT}: %-ltl/${OUTPUT_LTL}: RERS/%-ltl.go.formulae.txt RERS/%-ltl.go %-lt
 
 ${OUTPUT_REACH}: ${REACHOUTPUT}
 	@echo "model,isempty,spottime,ltsmintime,formulae" > $@
-	@tail -q -n +2 $< >> $@
+	@if [ ! -f $< ]; then\
+		tail -q -n +2 $< >> $@;\
+	fi
 
 ${REACHOUTPUT}: %-reach/${OUTPUT_REACH}: RERS/%-reach.go %-reach/output/go2pins-mc
 	@echo Processing $<.formulae.txt if it exists
-	@./run-reach.sh $@ $^ ${NBTHREADS}
+	@./run-reach.sh $@ $^ ${REACHTHREADS}
 	@echo End of processing $<
 
 %-reach/output/go2pins-mc: %-reach