Some diagnostics produced by g77 require sufficient explanation that the explanations are given below, and the diagnostics themselves identify the appropriate explanation.
Identification uses the GNU Info format—specifically, the info command that displays the explanation is given within square brackets in the diagnostic. For example:
foo.f:5: Invalid statement [info -f g77 M FOOEY]
More details about the above diagnostic is found in the g77 Info documentation, menu item M, submenu item FOOEY, which is displayed by typing the UNIX command info -f g77 M FOOEY.
Other Info readers, such as EMACS, may be just as easily used to display the pertinent node. In the above example, g77 is the Info document name, M is the top-level menu item to select, and, in that node (named Diagnostics, the name of this chapter, which is the very text you're reading now), FOOEY is the menu item to select.