anchor.diff:       ! read nclasses
        call parse_next_line(unt, line, parsed_line, nfields, err)
        if (err < 0) then
  -        write(0,*) "input file can be converted to a valid checkpoint file"
  +        write(0,*) "ERROR: invalid input file"
           stop
        end if
        read(parsed_line, *, iostat=err) nclasses
  
positive.code:      ! read data type
       call parse_next_line(unt, line, parsed_line, nfields, err)
       if (err < 0) then
          write(0,*) "input file can be converted to a valid checkpoint file"
          stop
       end if
       read(parsed_line, *, iostat=err) data_type
  
negative.code:            read(parsed_line, *) iv, prm(k + 1 : k + nclasses)
          else if (nfields == nclasses**2 + 2) then
             read(parsed_line, *) jv, iv
             if (jv > iv) then
                write(0,*) "ERROR: when reading couplings line p q {couplings}, it must be q > p"
                stop
             end if
             k = (2 * nvars - jv) * (jv - 1) / 2 + iv - jv
             k = nvars * nclasses + (k - 1) * nclasses**2
             read(parsed_line, *) jv, iv, prm(k + 1 : k + nclasses**2)
  