<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def Prepend(self, **kw):
 """Prepend values to existing construction variables in an Environment. """

        kw = copy_non_reserved_keywords(kw)
        for key, val in kw.items():
            # It would be easier on the eyes to write this using
            # "continue" statements whenever we finish processing an item,
            # but Python 1.5.2 apparently doesn't let you use "continue"
            # within try:-except: blocks, so we have to nest our code.
            try:
                orig = self._dict[key]
            except KeyError:
                # No existing variable in the environment, so just set
                # it to the new value.
                self._dict[key] = val
            else:
                try:
                    # Check if the original looks like a dictionary.
                    # If it is, we can't just try adding the value because
                    # dictionaries don't have __add__() methods, and
                    # things like UserList will incorrectly coerce the
                    # original dict to a list (which we don't want).
                    update_dict = orig.update
                except AttributeError:
                    try:
                        # Most straightforward:  just try to add them
                        # together.  This will work in most cases, when the
                        # original and new values are of compatible types.
                        self._dict[key] = val + orig
                    except (KeyError, TypeError):
                        try:
                            # Check if the added value is a list.
                            add_to_val = val.append
                        except AttributeError:
                            # The added value isn't a list, but the
                            # original is (by process of elimination),
                            # so insert the the new value in the original
                            # (if there's one to insert).
                            if val:
                                orig.insert(0, val)
                        else:
                            # The added value is a list, so append
                            # the original to it (if there's a value
                            # to append).
                            if orig:
                                add_to_val(orig)
                            self._dict[key] = val
                else:
                    # The original looks like a dictionary, so update it
                    # based on what we think the value looks like.
                    if SCons.Util.is_List(val):
                        for v in val:
                            orig[v] = None
                    else:
                        try:
                            update_dict(val)
                        except (AttributeError, TypeError, ValueError):
                            if SCons.Util.is_Dict(val):
                                for k, v in val.items():
                                    orig[k] = v
                            else:
                                orig[val] = None
        self.scanner_map_delete(kw)