// must at the least provide:
//
//	func (n *MyNode) String() string { return fmt.Sprint(n) }
//	func (n *MyNode) rawCopy() Node { c := *n; return &c }
//	func (n *MyNode) Format(s fmt.State, verb rune) { FmtNode(n, s, verb) }
//
// The embedding struct should also fill in n.op in its constructor,
// for more useful panic messages when invalid methods are called,
// instead of implementing Op itself.
type miniNode struct {