Ch2, Ex2

Its a necessary step because N is Free variable identifier and it may or may not exist in the environment of the caller. Hence to make sure of the presence of N, N-> 3 (in general mapping for all Free identifiers) is added to contextual environment.This is in terms with Lexical scoping or else it'll become dynamic scoping.

An example where N does not exist at time of call:

declare MulByN in
local N in
   N = 3
   proc {MulByN X ?Y}
      Y = N*X
   end
end
{Browse {MulByN 5 $}}

An exmple where N is not bound to 3 at the time of the call

declare MulByN N in
local N in
   N = 3
   proc {MulByN X ?Y}
      Y = N*X
   end
end
N = 6
{Browse {MulByN 5 $}}