diff --git a/README.md b/README.md
index 64262a0..d36c29b 100644
--- a/README.md
+++ b/README.md
@@ -15,6 +15,8 @@ cross-platform.
 Its major disadvantage is that it mostly doesn't exist yet.
 
 [![Build Status](https://travis-ci.org/zellyn/diskii.svg?branch=master)](https://travis-ci.org/zellyn/diskii)
+[![Report Card](https://goreportcard.com/badge/github.com/zellyn/diskii)](https://goreportcard.com/report/github.com/zellyn/diskii)
+
 
 It rhymes with “whiskey”.
 
diff --git a/cmd/catalog.go b/cmd/catalog.go
index 1ea70e1..1820afd 100644
--- a/cmd/catalog.go
+++ b/cmd/catalog.go
@@ -8,8 +8,6 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/zellyn/diskii/lib/disk"
-	_ "github.com/zellyn/diskii/lib/dos3"
-	_ "github.com/zellyn/diskii/lib/supermon"
 )
 
 var shortnames bool // flag for whether to print short filenames
diff --git a/cmd/delete.go b/cmd/delete.go
index 7c1733c..a4df421 100644
--- a/cmd/delete.go
+++ b/cmd/delete.go
@@ -8,8 +8,6 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/zellyn/diskii/lib/disk"
-	_ "github.com/zellyn/diskii/lib/dos3"
-	_ "github.com/zellyn/diskii/lib/supermon"
 )
 
 var missingok bool // flag for whether to consider deleting a nonexistent file okay
diff --git a/cmd/dump.go b/cmd/dump.go
index 9955671..78b56a9 100644
--- a/cmd/dump.go
+++ b/cmd/dump.go
@@ -8,8 +8,6 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/zellyn/diskii/lib/disk"
-	_ "github.com/zellyn/diskii/lib/dos3"
-	_ "github.com/zellyn/diskii/lib/supermon"
 )
 
 // dumpCmd represents the dump command, used to dump the raw contents
diff --git a/cmd/filetypes.go b/cmd/filetypes.go
index 768b2ac..5138217 100644
--- a/cmd/filetypes.go
+++ b/cmd/filetypes.go
@@ -8,8 +8,6 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/zellyn/diskii/lib/disk"
-	_ "github.com/zellyn/diskii/lib/dos3"
-	_ "github.com/zellyn/diskii/lib/supermon"
 )
 
 var all bool // flag for whether to show all filetypes
diff --git a/cmd/put.go b/cmd/put.go
index f281cc7..af6860b 100644
--- a/cmd/put.go
+++ b/cmd/put.go
@@ -8,9 +8,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/zellyn/diskii/lib/disk"
-	_ "github.com/zellyn/diskii/lib/dos3"
 	"github.com/zellyn/diskii/lib/helpers"
-	_ "github.com/zellyn/diskii/lib/supermon"
 )
 
 var filetypeName string // flag for file type
diff --git a/lib/disk/disk.go b/lib/disk/disk.go
index 5df5d3b..fc4e3c9 100644
--- a/lib/disk/disk.go
+++ b/lib/disk/disk.go
@@ -11,8 +11,9 @@ import (
 	"strings"
 )
 
+// Various DOS33 disk characteristics.
 const (
-	DOS33Tracks  = 35 // Tracks per disk
+	DOS33Tracks  = 35
 	DOS33Sectors = 16 // Sectors per track
 	// DOS33DiskBytes is the number of bytes on a DOS 3.3 disk.
 	DOS33DiskBytes  = 143360             // 35 tracks * 16 sectors * 256 bytes
@@ -39,6 +40,8 @@ type TrackSector struct {
 	Sector byte
 }
 
+// SectorDisk is the interface use to read and write disks by physical
+// (matches sector header) sector number.
 type SectorDisk interface {
 	// ReadPhysicalSector reads a single physical sector from the disk. It
 	// always returns 256 byes.
@@ -54,6 +57,8 @@ type SectorDisk interface {
 	Write(io.Writer) (int, error)
 }
 
+// LogicalSectorDisk is the interface used to read and write a disk by
+// *logical* sector number.
 type LogicalSectorDisk interface {
 	// ReadLogicalSector reads a single logical sector from the disk. It
 	// always returns 256 byes.
@@ -78,6 +83,8 @@ type MappedDisk struct {
 
 var _ LogicalSectorDisk = MappedDisk{}
 
+// NewMappedDisk returns a MappedDisk with the given
+// logical-to-physical sector mapping.
 func NewMappedDisk(sd SectorDisk, logicalToPhysical []byte) (MappedDisk, error) {
 	if logicalToPhysical != nil && len(logicalToPhysical) != int(sd.Sectors()) {
 		return MappedDisk{}, fmt.Errorf("NewMappedDisk called on a disk image with %d sectors per track, but a mapping of length %d", sd.Sectors(), len(logicalToPhysical))
diff --git a/lib/disk/dsk.go b/lib/disk/dsk.go
index 8312786..015a712 100644
--- a/lib/disk/dsk.go
+++ b/lib/disk/dsk.go
@@ -29,7 +29,7 @@ func LoadDSK(filename string) (DSK, error) {
 	}
 	// TODO(zellyn): handle 13-sector disks.
 	if len(bb) != DOS33DiskBytes {
-		return DSK{}, fmt.Errorf("Expected file %q to contain %d bytes, but got %d.", filename, DOS33DiskBytes, len(bb))
+		return DSK{}, fmt.Errorf("expected file %q to contain %d bytes, but got %d", filename, DOS33DiskBytes, len(bb))
 	}
 	return DSK{
 		data:             bb,
diff --git a/lib/disk/filetype.go b/lib/disk/filetype.go
index 27ee6f8..92a662e 100644
--- a/lib/disk/filetype.go
+++ b/lib/disk/filetype.go
@@ -11,6 +11,7 @@ import "fmt"
 // the ProDOS/SOS filetype byte definitions in the range 00-FF.
 type Filetype int
 
+// Filetypes.
 const (
 	FiletypeTypeless                Filetype = 0x00  //     | both   | Typeless file
 	FiletypeBadBlocks               Filetype = 0x01  //     | both   | Bad blocks file
diff --git a/lib/dos3/dos3.go b/lib/dos3/dos3.go
index 3c311e9..67293e6 100644
--- a/lib/dos3/dos3.go
+++ b/lib/dos3/dos3.go
@@ -19,6 +19,7 @@ const (
 	VTOCSector = 0
 )
 
+// DiskSector represents a track and sector.
 type DiskSector struct {
 	Track  byte
 	Sector byte
@@ -185,6 +186,8 @@ func (v *VTOC) FromSector(data []byte) error {
 	return nil
 }
 
+// DefaultVTOC returns a new, empty VTOC with values set to their
+// defaults.
 func DefaultVTOC() VTOC {
 	v := VTOC{
 		CatalogTrack:           0x11,
@@ -253,9 +256,9 @@ func (cs *CatalogSector) FromSector(data []byte) error {
 // Filetype is the type for dos 3.3 filetype+locked status byte.
 type Filetype byte
 
+// The DOS3 filetypes.
 const (
-	// Hex 80+file type - file is locked,
-	// Hex 00+file type - file is not locked.
+	// FiletypeLocked is just setting the high bit on other file types.
 	FiletypeLocked Filetype = 0x80
 
 	FiletypeText        Filetype = 0x00 // Text file
@@ -268,8 +271,10 @@ const (
 	FiletypeB           Filetype = 0x40 // B type file
 )
 
+// FileDescStatus is the type used to mark file descriptor status.
 type FileDescStatus int
 
+// The three actual file descriptor status values.
 const (
 	FileDescStatusNormal FileDescStatus = iota
 	FileDescStatusDeleted
diff --git a/lib/dos3/dos3_test.go b/lib/dos3/dos3_test.go
index 8575b2f..a287053 100644
--- a/lib/dos3/dos3_test.go
+++ b/lib/dos3/dos3_test.go
@@ -85,6 +85,9 @@ func TestReadCatalog(t *testing.T) {
 		t.Fatal(err)
 	}
 	fds, deleted, err := ReadCatalog(dsk)
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	fdsWant := []struct {
 		locked bool
diff --git a/lib/errors/errors.go b/lib/errors/errors.go
index e313f84..315ee07 100644
--- a/lib/errors/errors.go
+++ b/lib/errors/errors.go
@@ -9,7 +9,8 @@ import (
 	"fmt"
 )
 
-// Copy of errors.New, so you this package can be imported instead.
+// New is a copy of errors.New, so this package can be imported as a
+// replacement.
 func New(text string) error {
 	return errors.New(text)
 }
diff --git a/lib/helpers/helpers.go b/lib/helpers/helpers.go
index 6b41b32..1b85fe0 100644
--- a/lib/helpers/helpers.go
+++ b/lib/helpers/helpers.go
@@ -9,6 +9,8 @@ import (
 	"os"
 )
 
+// FileContentsOrStdIn returns the contents of a file, unless the file
+// is "-", in which case it reads from stdin.
 func FileContentsOrStdIn(s string) ([]byte, error) {
 	if s == "-" {
 		return ioutil.ReadAll(os.Stdin)
diff --git a/lib/supermon/supermon.go b/lib/supermon/supermon.go
index 785ed82..226ebef 100644
--- a/lib/supermon/supermon.go
+++ b/lib/supermon/supermon.go
@@ -473,7 +473,7 @@ func (st SymbolTable) DeleteSymbol(name string) bool {
 // already exists with a different address, it deletes it first.
 func (st SymbolTable) AddSymbol(name string, address uint16) error {
 	if address == 0 {
-		return fmt.Errorf("cannot set symbol %q to address 0")
+		return fmt.Errorf("cannot set symbol %q to address 0", name)
 	}
 	hash := addrHash(address)
 	pos := -1
@@ -584,7 +584,7 @@ func (st SymbolTable) ParseCompoundSymbol(name string) (address uint16, symAddre
 		if _, err := encodeSymbol(name); err != nil {
 			return 0, 0, name, nil
 		}
-		return 0, 0, "", fmt.Errorf("%q is not a valid symbol name or address")
+		return 0, 0, "", fmt.Errorf("%q is not a valid symbol name or address", name)
 	}
 
 	if parts[0] == "" {
@@ -630,7 +630,7 @@ func (st SymbolTable) FilesForCompoundName(filename string) (numFile byte, named
 		return 0, 0, "", fmt.Errorf("invalid file number: %q", parts[0])
 	}
 	if numFile2 := parseAddressFilename(parts[1]); numFile2 != 0 {
-		return 0, 0, "", fmt.Errorf("cannot valid file number (%q) as a filename")
+		return 0, 0, "", fmt.Errorf("cannot use valid file number (%q) as a filename", parts[1])
 	}
 	namedFile, err = st.FileForName(parts[1])
 	if err != nil {
diff --git a/main.go b/main.go
index 202f506..ca9cd99 100644
--- a/main.go
+++ b/main.go
@@ -2,7 +2,13 @@
 
 package main
 
-import "github.com/zellyn/diskii/cmd"
+import (
+	"github.com/zellyn/diskii/cmd"
+
+	// Import disk operator factories for DOS3 and Super-Mon
+	_ "github.com/zellyn/diskii/lib/dos3"
+	_ "github.com/zellyn/diskii/lib/supermon"
+)
 
 func main() {
 	cmd.Execute()