anchor.diff:  
       ! Translate debug_c (zero-based verbosity indicator) approximately
       ! to print_control (general signed integer with meaning attached to
  -    ! certain values)
  +    ! certain values).  TODO consider debug vs. print_control.
       print_control = debug_c - 1
   
       ! Initialize the state and task
  
positive.code:      ! 'status_message_length': Usable length of 'status_message_c'
       !    buffer.  Recommend at least 100.
       !
       ! 'status_message': Returns a message explaining the exit status.
       !
       ! 'status': Returns the exit status code, one of the
       !    LBFGSB_STATUS_* constants defined in enumeration above.
  
negative.code:     ! Locals
      integer :: length
  
      ! Find the length of the shorter string.  Leave room for a
      ! terminating null character.
      length = min(len(string_f), string_c_length - 1)
      ! Copy no more than 'length' characters
      string_c = string_f(1:length)
      ! Fill rest of string (at least one character) with nulls
      string_c(length+1:string_c_length) = c_null_char
    end subroutine convert_f_c_string
  
  