Graph's a bit too big for mermaid though.

#!/bin/bash

grep -E '^[^C].*(CALL|SUBROUTINE)' bulletin*.for \

  | sed 's/ *! .*//;s/:.*\(CALL\|SUBROUTINE\) / \1 /;s/(.*//' \

  | awk '

        BEGIN {

          e="flowchart LR"

        }

        {

          if ($1 != f) {

            print e;

            sg=$1; gsub(".for", "", sg);

            print "  subgraph " sg;

            print "    direction LR";

            e="  end";

          }

        }

        $2 == "SUBROUTINE" {

          f=$1; s=$3;

          print "    " s "(" s ")";

        }

        $2 == "CALL" {

          if ($1 != f) {

            f=$1; s="main";

            print "    " s "(" s ")";

          }

          print "    " s " --> " $3;

        }

        END {

          print e;

        }

        ' \

  | sed 's/\$/__/g'