Comments out of order after adding item to Go AST

The following test attempts to use AST to add fields to a struct. The fields are added correctly, but the comments are added out of order. I gather the position may need to be specified manually, but I’ve so far drawn a blank finding an answer.

Here’s a failing test: http://play.golang.org/p/RID4N30FZK

Here’s the code:

package generator

import (
    "bytes"
    "fmt"
    "go/ast"
    "go/parser"
    "go/printer"
    "go/token"
    "testing"
)

func TestAst(t *testing.T) {

    source := `package a

// B comment
type B struct {
    // C comment
    C string
}`

    fset := token.NewFileSet()
    file, err := parser.ParseFile(fset, "", []byte(source), parser.ParseComments)
    if err != nil {
        t.Error(err)
    }

    v := &visitor{
        file: file,
    }
    ast.Walk(v, file)

    var output []byte
    buf := bytes.NewBuffer(output)
    if err := printer.Fprint(buf, fset, file); err != nil {
        t.Error(err)
    }

    expected := `package a

// B comment
type B struct {
    // C comment
    C string
    // D comment
    D int
    // E comment
    E float64
}
`

    if buf.String() != expected {
        t.Error(fmt.Sprintf("Test failed. Expected:\n%s\nGot:\n%s", expected, buf.String()))
    }

    /*
    actual output = `package a

// B comment
type B struct {
    // C comment
    // D comment
    // E comment
    C   string
    D   int
    E   float64
}
`
    */

}

type visitor struct {
    file *ast.File
}

func (v *visitor) Visit(node ast.Node) (w ast.Visitor) {

    if node == nil {
        return v
    }

    switch n := node.(type) {
    case *ast.GenDecl:
        if n.Tok != token.TYPE {
            break
        }
        ts := n.Specs[0].(*ast.TypeSpec)
        if ts.Name.Name == "B" {
            fields := ts.Type.(*ast.StructType).Fields
            addStructField(fields, v.file, "int", "D", "D comment")
            addStructField(fields, v.file, "float64", "E", "E comment")
        }
    }

    return v
}

func addStructField(fields *ast.FieldList, file *ast.File, typ string, name string, comment string) {
    c := &ast.Comment{Text: fmt.Sprint("// ", comment)}
    cg := &ast.CommentGroup{List: []*ast.Comment{c}}
    f := &ast.Field{
        Doc:   cg,
        Names: []*ast.Ident{ast.NewIdent(name)},
        Type:  ast.NewIdent(typ),
    }
    fields.List = append(fields.List, f)
    file.Comments = append(file.Comments, cg)
}