Fortran77 vs Fortran90 were fairly different languages that required a substantial revision to the numerical methods assignments that I had in the early 90s as the department shifted from one to the other.
> There are now two forms of the source code. The old source code form, which is based on the punched card, and now called fixed form and the new free form.
> ...
> A completely new capability of Fortran 90 is recursion. Note that it requires that you assign a new property RESULT to the output variable in the function declaration. This output variable is required inside the function as the "old" function name in order to store the value of the function. At the actual call of the function, both externally and internally, you use the outer or "old" function name. The user can therefore ignore the output variable.
https://www.nsc.liu.se/~boein/f77to90/f77to90.html
> There are now two forms of the source code. The old source code form, which is based on the punched card, and now called fixed form and the new free form.
> ...
> A completely new capability of Fortran 90 is recursion. Note that it requires that you assign a new property RESULT to the output variable in the function declaration. This output variable is required inside the function as the "old" function name in order to store the value of the function. At the actual call of the function, both externally and internally, you use the outer or "old" function name. The user can therefore ignore the output variable.