In Luna you can spread a command line over multiple lines. For this, add an underscore at the corresponding position. The preprocessor adds these lines automatically together while building.

buffer.WordValue(TCP_packet+TCP_Checksum) = Ce16(_
  TcpChecksum(_
  buffer,_
  eTHeRNeT_HeADeR_LeNGTH+IP_HeADeR_LeNGTH-IP_PSeUDOHeADeR_LeNGTH,_
  TCP_HeADeR_LeNGTH+IP_PSeUDOHeADeR_LeNGTH+dlen)_
  )

procedure MakeTCPheaderBuffer(_
  byRef buffer as MemoryBlock,_
  socket as byte,_
  TCP_flags as byte,_
  dlen as word,_
  Windowsize as word)
[..]
endproc