A more modern version of Fortran than Fortran 77. All Fortran 77 programs are valid Fortran 90, but Fortran 90 source code
can also be free format. I.e., a line of code can begin in column 1, whereas in Fortran 77 it had to begin in column 7 or later.
Comments in free format Fortran 90 begin with an exclamation mark; anything from an exclamation mark to the end of the line
is treated as a comment. Other new features in Fortran 90 are allocatable arrays, user-defined data structures and new control
structures such as a new form of the DO loop.