func (h *ErrorVector) GetError(mode int) error {
	if len(h.errors) == 0 {
		return nil
	}
	return h.GetErrorList(mode)
}
GetError is like GetErrorList, but it returns an os.Error instead
so that a nil result can be assigned to an os.Error variable and
remains nil.
package scanner
import (
	"fmt"
	"io"
	"sort"
	"github.com/rogpeppe/godef/go/token"
)
type ErrorHandler interface {
	Error(pos token.Position, msg string)
}
type ErrorVector struct {
	errors []*Error
}
func (h *ErrorVector) Reset() { h.errors = h.errors[:0] }
func (h *ErrorVector) ErrorCount() int { return len(h.errors) }
type Error struct {
	Pos token.Position
	Msg string
}
func (e *Error) Error() string {
	if e.Pos.Filename != "" || e.Pos.IsValid() {
		return e.Pos.String() + ": " + e.Msg
	}
	return e.Msg
}
type ErrorList []*Error
func (p ErrorList) Len() int      { return len(p) }
func (p ErrorList) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
func (p ErrorList) Less(i, j int) bool {
	e := &p[i].Pos
	f := &p[j].Pos
	if e.Filename < f.Filename {
		return true
	}
	if e.Filename == f.Filename {
		if e.Line < f.Line {
			return true
		}
		if e.Line == f.Line {
			return e.Column < f.Column
		}
	}
	return false
}
func (p ErrorList) Error() string {
	switch len(p) {
	case 0:
		return "unspecified error"
	case 1:
		return p[0].Error()
	}
	return fmt.Sprintf("%s (and %d more errors)", p[0].Error(), len(p)-1)
}
const (
	Raw         = iota  
	Sorted              
	NoMultiples         
)
func (h *ErrorVector) GetErrorList(mode int) ErrorList {
	if len(h.errors) == 0 {
		return nil
	}
	list := make(ErrorList, len(h.errors))
	copy(list, h.errors)
	if mode >= Sorted {
		sort.Sort(list)
	}
	if mode >= NoMultiples {
		var last token.Position  
		i := 0
		for _, e := range list {
			if e.Pos.Filename != last.Filename || e.Pos.Line != last.Line {
				last = e.Pos
				list[i] = e
				i++
			}
		}
		list = list[0:i]
	}
	return list
}