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