Wireshark Lua API: How to get TCP header info?

I'm writing a dissector ^{(to be added to DissectorTable.get("tcp.port"))} for a TCP-based application. I've gone through the Wireshark API doc but could not find out how to get TCP header's info like

• SYN/ACK flags
• Sequence number
• ACK'ed sequence number

UPDATE:

Based on the answer I'd put example snippet here. Just FYI.

local proto = Proto("myproto", "my proto")

-- ...
-- ...

--
-- A Field object can only be created *outside* of the callback
-- functions of dissectors, post-dissectors, heuristic-dissectors,
-- and taps.
--
local F_tcp_seq_rel = Field.new('tcp.seq')      -- relative seq num
local F_tcp_seq_raw = Field.new('tcp.seq_raw')  -- raw seq num

function proto.dissector(tvbuf, pinfo, tree)

    -- ...
    -- ...

    local seq_rel = F_tcp_seq_rel()  -- yes the Field object is callable!
    local seq_raw = F_tcp_seq_raw()

    -- ...
    -- ...
end

DissectorTable.get("tcp.port"):add(12345, proto)

The way to get any field data, TCP or otherwise, is via a Field Extractor. So for example:

local tcp_flags_syn = Field.new("tcp.flags.syn")
local tcp_flags_ack = Field.new("tcp.flags.ack")

-- If you want relative sequence/acknowledgment numbers:
local tcp_seq = Field.new("tcp.seq")
local tcp_ack = Field.new("tcp.ack")

-- If you want absolute sequence/acknowledgment numbers:
local tcp_seq_raw = Field.new("tcp.seq_raw")
local tcp_ack_raw = Field.new("tcp.ack_raw")

If you need additional help using these fields, you may want to look at some of the Lua examples provided on the Wireshark Examples and/or Contrib wiki pages.