func (b *Builder) Build() []byte {
	b.w.WriteByte(')')  
	return b.w.Bytes()
}
Build finalizes package being built.
Package bytes returned.
It is illegal to call Build method twice one the same builder.
package export
import (
	"backends/lapc"
	"sexp"
	"tu"
)
type Builder struct {
	w writer
}
func NewBuilder(pkg *tu.Package) *Builder {
	b := &Builder{}
	w := &b.w
	w.WriteByte('(')  
	w.WriteSymbol(pkg.Name)
	w.WriteString(pkg.Comment)
	return b
}