"... it is very seldom that in these circumstances a sentence of much less than seven years would be appropriate, the purpose of the sentence of that length being threefold: first of all to deter the offender from behaving in this way again; secondly to deter others from arming themselves with weapons and holding up banks; and thirdly, a matter which is sometimes overlooked, to punish the offender for having carried out this wicked crime and having put several people in fear of death."