SYNOPSIS
  int CantSee ()
  int CantSee (object env)

IMPLEMENTED
  All seeing objects.

DESCRIPTION
  Checks if the called object can see in its current environment,
  or <env> if given.
  It has to return:
    < 0: it is too dark
    = 0: the object can see.
    > 0: is is too bright.

  Called by kernel objects.

