theorem Th31:
  m * n = 65 implies m = 1 & n = 65
                  or m = 5 & n = 13
                  or m = 13 & n = 5
                  or m = 65 & n = 1
  proof
    assume
A1: m*n = 65;
    m divides m*n;
    then m = 1 or m = 5 or m = 13 or m = 65 by A1,Th17;
    hence thesis by A1;
  end;
