ftc acts on a file in place, inserting comment characters (c;)
in front of those lines which are deemed to be inactive in the current
environment, and removing any such disabling comment markers from active
segments.
It is important to realise that ftc is unlike other preprocessors, such
as cpp, in that it acts symmetrically, transforming a file from one
environment to another without loss of information. Both the initial and final
file could be given to the fortran compiler, and if ftc is run on a
file which is already correct for the current environment, no changes result.
ftc does not even change the modification time of the file, in order
for make to work sensibly, and to help with version control.
The syntax for running ftc is
ftc [-D name] [-U name] [-l|-u|-m] [-h string] [-t string] file1 file2 ...
name, so that code marked as being
active if name is set (see below) is uncommented if necessary.
name to false, to force commenting out of
code which is dependent on it. Note that all flags apart from a subset
(defined when ftc is built) are false by default.
P.J. Knowles and H.-J. Werner