Why Fortran 90 is much better than Fortran 77

I'd like to digress from our geophysics-mathematics themes to explain why Fortran 90 has been a great step forward over Fortran 77. All the illustrations in this book were originally computed in F77. Then module solver_tiny was simply a subroutine. It was not one module for the whole book, as it is now, but it was many conceptually identical subroutines, dozens of them, one subroutine for each application. The reason for the proliferation was that F77 lacks the ability of F90 to represent operators as having two ways to enter, one for science and another for math. On the other hand, F77 did not require the half a page of definitions that we see here in F90. But the definitions are not difficult to understand, and they are a clutter that we must see once and never again. Another benefit is that the book in F77 had no easy way to switch from the cgstep solver to other solvers.