A Sudoku program in Fortran 95

A remarkable craze swept though many countries in the year 2005. Already known and popular in Japan, sudoku was introduced into the UK and thence into other European countries and the USA. Its rules are simple. Given a 9 x 9 grid with some numbers already in place, the solver has to fill in the missing values such that, in each row, each column and each 3 x 3 box, the digits 1 to 9 appear once and once only. The starting digits in this example are arranged symmetrically, which is merely a matter of taste. However, it is widely accepted that a properly formed puzzle has a unique solution and that it can thus be solved, without having to make any guesses, by pure logic.An excellent source of further details is the Wikipedia article at http://en.wikipedia.org/wiki/Sudoku