X       Expr      // expression
		Lbrack  token.Pos // position of "["
		Indices []Expr    // index expressions
		Rbrack  token.Pos // position of "]"
	}

	// A SliceExpr node represents an expression followed by slice indices.
	SliceExpr struct {
		X      Expr      // expression
		Lbrack token.Pos // position of "["
		Low    Expr      // begin of slice range; or nil
		High   Expr      // end of slice range; or nil

	// a valid type.
	if x.typ == nil {
		x.typ = Typ[Invalid]
	}

	check.index(index, length)
	return false
}

func (check *Checker) sliceExpr(x *operand, e *ast.SliceExpr) {
	check.expr(nil, x, e.X)
	if x.mode == invalid {
		check.use(e.Low, e.High, e.Max)
		return
	}

	valid := false
	length := int64(-1) // valid if >= 0

	// a valid type.
	if x.typ == nil {
		x.typ = Typ[Invalid]
	}

	check.index(index, length)
	return false
}

func (check *Checker) sliceExpr(x *operand, e *syntax.SliceExpr) {
	check.expr(nil, x, e.X)
	if x.mode == invalid {
		check.use(e.Index[:]...)
		return
	}

	valid := false
	length := int64(-1) // valid if >= 0

		children = append(children,
			tok(n.Select, len("select")))

	case *ast.SelectorExpr:
		// nop

	case *ast.SendStmt:
		children = append(children,
			tok(n.Arrow, len("<-")))

	case *ast.SliceExpr:
		children = append(children,
			tok(n.Lbrack, len("[")),
			tok(n.Rbrack, len("]")))

	case *ast.StarExpr:
		children = append(children, tok(n.Star, len("*")))

	case *ast.StructType:

		f.walk(&n.X, context, visit)
	case *ast.SelectorExpr:
		f.walk(&n.X, ctxSelector, visit)
	case *ast.IndexExpr:
		f.walk(&n.X, ctxExpr, visit)
		f.walk(&n.Index, ctxExpr, visit)
	case *ast.SliceExpr:
		f.walk(&n.X, ctxExpr, visit)
		if n.Low != nil {
			f.walk(&n.Low, ctxExpr, visit)
		}
		if n.High != nil {
			f.walk(&n.High, ctxExpr, visit)
		}
		if n.Max != nil {

	case *ast.IndexExpr:
		a.apply(n, "X", nil, n.X)
		a.apply(n, "Index", nil, n.Index)

	case *ast.IndexListExpr:
		a.apply(n, "X", nil, n.X)
		a.applyList(n, "Indices")

	case *ast.SliceExpr:
		a.apply(n, "X", nil, n.X)
		a.apply(n, "Low", nil, n.Low)
		a.apply(n, "High", nil, n.High)
		a.apply(n, "Max", nil, n.Max)

	case *ast.TypeAssertExpr:
		a.apply(n, "X", nil, n.X)

	case ir.OSTRINGHEADER:
		n := n.(*ir.StringHeaderExpr)
		return walkStringHeader(n, init)

	case ir.OSLICE, ir.OSLICEARR, ir.OSLICESTR, ir.OSLICE3, ir.OSLICE3ARR:
		n := n.(*ir.SliceExpr)
		return walkSlice(n, init)

	case ir.ONEW:
		n := n.(*ir.UnaryExpr)
		return walkNew(n, init)

	case ir.OADDSTR:
		return walkAddString(n.(*ir.AddStringExpr), init)

	case ir.OAPPEND:

		walkBeforeAfter(&n.X, before, after)
		walkBeforeAfter(&n.Index, before, after)
	case *ast.IndexListExpr:
		walkBeforeAfter(&n.X, before, after)
		walkBeforeAfter(&n.Indices, before, after)
	case *ast.SliceExpr:
		walkBeforeAfter(&n.X, before, after)
		if n.Low != nil {
			walkBeforeAfter(&n.Low, before, after)
		}
		if n.High != nil {
			walkBeforeAfter(&n.High, before, after)
		}
	case *ast.TypeAssertExpr:

				typeof[n] = typeof[n.X]
				return
			}
			// x.(T) has type T.
			if t := typeof[n.Type]; isType(t) {
				typeof[n] = getType(t)
			} else {
				typeof[n] = gofmt(n.Type)
			}

		case *ast.SliceExpr:
			// x[i:j] has type of x.
			typeof[n] = typeof[n.X]

		case *ast.IndexExpr:
			// x[i] has key type of x's type.
			t := expand(typeof[n.X])

		p.print(_Rbrace)

	case *ParenExpr:
		p.print(_Lparen, n.X, _Rparen)

	case *SelectorExpr:
		p.print(n.X, _Dot, n.Sel)

	case *IndexExpr:
		p.print(n.X, _Lbrack, n.Index, _Rbrack)

	case *SliceExpr:
		p.print(n.X, _Lbrack)
		if i := n.Index[0]; i != nil {
			p.printNode(i)
		}
		p.print(_Colon)
		if j := n.Index[1]; j != nil {
			p.printNode(j)
		}
		if k := n.Index[2]; k != nil {