func (f *File) Seq(name string) (*Seq, error) {
	rec, ok := f.Index[name]
	if !ok {
		return nil, errors.New("fai: no sequence")
	}
	return &Seq{r: f.r, Record: rec, end: rec.Length}, nil
}
Seq returns a handle to the complete sequence identified by the given name.
package fai
import (
	"errors"
	"io"
)
type File struct {
	Index
	r io.ReaderAt
}
func NewFile(r io.ReaderAt, idx Index) *File {
	return &File{r: r, Index: idx}
}