anchor.diff:  
   subroutine gcmdln()
   
  -use io, only: stderr, verbosity
  +use io, only: stderr, verbosity, isNumeric
   
   use grid, only: output_file, &
                   grid_mode, &
  
positive.code:             cycle
          else
              call get_command_argument(i,tag)
              read (tag,*,iostat=ios) x_end
              if (ios.ne.0) then
                  ! No second value provided
  
negative.code:         ! First argument (starting x value) is required
          i = i + 1
          call get_command_argument(i,tag)
          if (.not.isNumeric(tag)) call usage('grid: x_beg is not numeric')
          read (tag,*) x_beg
          isXDefined = .true.
          ! Second argument (ending x value) is optional
  