if [ -e $3 ]
then
  echo '?' $3
  exit
fi
if ! grep -q 'REACTION PATH' $1
then
  echo '?' $1
  exit
fi
if ! grep -q 'REACTION PATH' $2
then
  echo '?' $2
  exit
fi
sed -n '/REAC/,$p' $1  > $3
sed -n '/REAC/,$p' $2 >> $3