By way of illustration, below is the in-code documentation for MIRIAD subroutine axistype, which uses the ``directives'' noted previously.
c* Axistype - Find the axis label and plane value in user friendly units
c& mchw
c: plotting
c+
subroutine AxisType(lIn,axis,plane,ctype,label,value,units)
c
implicit none
integer lIn,axis,plane
character ctype*9,label*13,units*13
double precision value
c
c Find the axis label and plane value in user friendly units.
c
c Inputs:
c lIn The handle of the image.
c axis The image axis.
c plane The image plane along this axis.
c Output:
c ctype The official ctype for the input axis.
c label A nice label for this axis.
c value The value at the plane along this axis.
c units User friendly units for this axis.
c--
Note that the programmer has woven executable code into the documentation
(the lines that are not commented out): anything between the c+ and
the c-- is considered to be part of the documentation, even though
the lines are actually part of the subroutine code itself.
A subroutine source code file (or a program source code file) may contain multiple subroutines, each documented as above.