What does ".=" in vim scripts mean?

I've often seen assignments to variables of the form "let s.='something'" Here's the specific piece of code in a vim script that I've been struggling to understand:

let s .= '%' . i . 'T'
let s .= (i == t ? '%1*' : '%2*')
let s .= ' '
let s .= i . ':'
let s .= winnr . '/' . tabpagewinnr(i,'$')
let s .= ' %*'
let s .= (i == t ? '%#TabLineSel#' : '%#TabLine#')

The code adds the tab number (i) and viewport number (winnr of tabpagewinnr(i,'$')) to the tab name, so that it looks something like "1: 2/4 Buffer name". From 开发者_如何学Gothe looks of it, the .= operation seems to be appending stuff to s. But then, I don't understand what the first two lines do. Any help is appreciated.

vim's online help is your friend:

:h .=

 :let {var} .= {expr1}    Like ":let {var} = {var} . {expr1}".

:h expr-.

 expr6 .   expr6 ..   String concatenation

:h expr1 (well - this is a little hard to find):

 expr2 ? expr1 : expr1

 The expression before the '?' is evaluated to a number.  If it evaluates to TRUE, the result is the value of the expression between the '?' and ':', otherwise the result is the value of the expression after the ':'.

 Example:
   :echo lnum == 1 ? "top" : lnum

  Since the first expression is an "expr2", it cannot contain another ?:.  The
  other two expressions can, thus allow for recursive use of ?:.
  Example:
    :echo lnum == 1 ? "top" : lnum == 1000 ? "last" : lnum

  To keep this readable, using |line-continuation| is suggested:
    :echo lnum == 1
    :\  ? "top"
    :\  : lnum == 1000
    :\      ? "last"
    :\      : lnum

  You should always put a space before the ':', otherwise it can be mistaken for
  use in a variable such as "a:1".

One at a time:

let s .= '%' . i . 'T'

Assuming i=9 and s="bleah", s will now be "bleah%9T"

let s .= (i == t ? '%1*' : '%2*')

This is the familiar ternary operator from C. If t==9, then s is now "bleah%9T%1*". If t is anything but 9, then s is now "bleah%9T%2*"