How to use the schema.mutable.Selection function in schema

if alt and key == 'r':
            target = mkstruct(u"null")
            block = mkstruct(u"return:value", target)
            return self.insert_inline_deep(target, block, expressions)
        if text == '(':
            target = mkstruct(u"null")
            block = mkstruct(u"call:callee:arguments", target, mutable.List([]))
            return self.insert_inline(target, block, expressions)
        if key == 'space':
            self.selection = next_selection(mutable.get_object(selection))
            overlay.dirty = True
        elif len(text) > 0 and not self.insert_text(text):
            if text.isalpha():
                target = mutable.String(u"")
                if self.insert_object(mkstruct(u'variable:name', target)):
                    self.selection = mutable.Selection(target, 0)
                    self.insert_text(text)
            elif text.isdigit():
                target = mutable.String(u"")
                if self.insert_object(mkstruct(u'number:value', target)):
                    self.selection = mutable.Selection(target, 0)
                    self.insert_text(text)

def enter_head(obj):
    if mutable.isstruct(obj) and len(obj) > 0:
        return enter_head(obj[0])
    if mutable.isstruct(obj):
        return obj
    else:
        return mutable.Selection(obj, 0)

if isinstance(obj, mutable.Selection):
        if mutable.iscont(obj.container) and obj.head > 0:
            return enter_tail(obj.container[obj.head-1])
        else:
            obj = obj.container
    if isinstance(obj, mutable.Document):
        return mutable.Selection(obj, 0)
    index = obj.parent.index(obj)
    if mutable.isstruct(obj.parent):
        if index > 0:
            nxt = obj.parent[index-1]
            return enter_tail(nxt)
        else:
            return walk_prev_selection(obj.parent)
    else:
        return mutable.Selection(obj.parent, index)

def next_selection(obj):
    if isinstance(obj, mutable.Document):
        return mutable.Selection(obj, len(obj))
    index = obj.parent.index(obj)
    if mutable.isstruct(obj.parent):
        if index + 1 < len(obj.parent):
            nxt = obj.parent[index+1]
            if mutable.islist(nxt):
                return mutable.Selection(nxt, 0)
            else:
                return nxt
        else:
            return next_selection(obj.parent)
    else:
        return mutable.Selection(obj.parent, index+1)

def next_selection(obj):
    if isinstance(obj, mutable.Document):
        return mutable.Selection(obj, len(obj))
    index = obj.parent.index(obj)
    if mutable.isstruct(obj.parent):
        if index + 1 < len(obj.parent):
            nxt = obj.parent[index+1]
            if mutable.islist(nxt):
                return mutable.Selection(nxt, 0)
            else:
                return nxt
        else:
            return next_selection(obj.parent)
    else:
        return mutable.Selection(obj.parent, index+1)

def walk_next_selection(obj):
    if isinstance(obj, mutable.Selection):
        if mutable.iscont(obj.container) and obj.head < obj.last:
            return enter_head(obj.container[obj.head])
        else:
            obj = obj.container
    if isinstance(obj, mutable.Document):
        return mutable.Selection(obj, len(obj))
    index = obj.parent.index(obj)
    if mutable.isstruct(obj.parent):
        if index + 1 < len(obj.parent):
            nxt = obj.parent[index+1]
            return enter_head(nxt)
        else:
            return walk_next_selection(obj.parent)
    else:
        return mutable.Selection(obj.parent, index+1)

def walk_next_selection(obj):
    if isinstance(obj, mutable.Selection):
        if mutable.iscont(obj.container) and obj.head < obj.last:
            return enter_head(obj.container[obj.head])
        else:
            obj = obj.container
    if isinstance(obj, mutable.Document):
        return mutable.Selection(obj, len(obj))
    index = obj.parent.index(obj)
    if mutable.isstruct(obj.parent):
        if index + 1 < len(obj.parent):
            nxt = obj.parent[index+1]
            return enter_head(nxt)
        else:
            return walk_next_selection(obj.parent)
    else:
        return mutable.Selection(obj.parent, index+1)

def enter_tail(obj):
    if mutable.isstruct(obj) and len(obj) > 0:
        return enter_tail(obj[len(obj)-1])
    if mutable.isstruct(obj):
        return obj
    else:
        return mutable.Selection(obj, len(obj))

def prev_selection(obj):
    if isinstance(obj, mutable.Document):
        return mutable.Selection(obj, 0)
    index = obj.parent.index(obj)
    if mutable.isstruct(obj.parent):
        if index > 0:
            nxt = obj.parent[index-1]
            if mutable.islist(nxt):
                return mutable.Selection(nxt, len(nxt))
            else:
                return nxt
        else:
            return prev_selection(obj.parent)
    else:
        return mutable.Selection(obj.parent, index)

def on_mousedown(button, pos):
    if frame.mode is None or frame.mode.on_mousedown(button, pos):
        intron = frame.pick_intron(pos)
        if intron is None:
            set_mode( None )
            return
        if isinstance(intron.controller, LayoutController):
            obj = intron.controller.obj
            if isinstance(obj, (mutable.List, mutable.String, mutable.Document)):
                head = intron.pick_offset(pos)
                set_mode( EditMode(frame, mutable.Selection(obj, head)) )
            else:
                set_mode( EditMode(frame, mutable.normalize(obj)) )
        # replace EditMode with grabber later.