[Haskell-beginners] do not understand Semantics of Case Expressions in report

[Michael Mekhanoshin]

at http://www.haskell.org/onlinereport/exps.html

on figure Figure 4 (Semantics of Case Expressions, Part 2)

one can see:

(m)      case  v  of {  K  { f1  =  p1  ,  f2  =  p2  ,  ... } ->  e ; _ -
>  e'  }
       =  case e' of {
          y ->
           case  v  of {
             K  {  f1  =  p1  } ->
                   case  v  of { K  { f2  =  p2  ,  ...  } ->  e ; _ ->
y  };
                   _ ->  y  }}
       where f1, f2, ... are fields of constructor K; y is a new variable

(In Figures 3.1--3.2: e, e' and ei are expressions; g and gi are
boolean-valued expressions; p and pi are patterns; v, x, and xi are
variables; K and K' are algebraic datatype (data) constructors
(including tuple constructors); and N is a newtype constructor.)


I'm totally confused with this identity. Especialy with " =  case e'".
Can anyone explain a little what does this rule stand, please?
It is better to see some example.

Respect,
Michael