diff --git a/src/p_lx_elf.cpp b/src/p_lx_elf.cpp
index 296ef899..fa55470f 100644
--- a/src/p_lx_elf.cpp
+++ b/src/p_lx_elf.cpp
@@ -1978,7 +1978,7 @@ bool PackLinuxElf32::calls_crt1(Elf32_Rel const *rel, int sz)
 #include "p_elf_enum.h"
 #undef WANT_REL_ENUM
 
-int PackLinuxElf32::canUnpack()  // really 'bool'
+int PackLinuxElf32::canUnpack() // bool, except -1: format known, but not packed
 {
     if (checkEhdr(&ehdri)) {
         return false;
@@ -2409,7 +2409,7 @@ proceed: ;
     return true;
 }
 
-int PackLinuxElf64::canUnpack()  // really 'bool'
+int PackLinuxElf64::canUnpack() // bool, except -1: format known, but not packed
 {
     if (checkEhdr(&ehdri)) {
         return false;
@@ -2497,7 +2497,7 @@ PackLinuxElf64::canPack()
             max_offset = UPX_MAX(max_offset, get_te64(&phdr->p_filesz) + get_te64(&phdr->p_offset));
         }
     }
-    if (canUnpack()) {
+    if (canUnpack() > 0) {
         throwAlreadyPacked();
     }
     // We want to compress position-independent executable (gcc -pie)
@@ -5340,7 +5340,7 @@ PackLinuxElf32x86::~PackLinuxElf32x86()
 {
 }
 
-int PackLinuxElf32x86::canUnpack()  // really 'bool'
+int PackLinuxElf32x86::canUnpack() // bool, except -1: format known, but not packed
 {
     if (super::canUnpack()) {
         return true;
diff --git a/src/p_lx_elf.h b/src/p_lx_elf.h
index 3258f609..1f3bc14d 100644
--- a/src/p_lx_elf.h
+++ b/src/p_lx_elf.h
@@ -42,7 +42,7 @@ public:
     /*virtual void buildLoader(const Filter *);*/
     virtual int getVersion() const override { return 14; } // upx-3.96 cannot upack, for instance
     virtual bool canUnpackVersion(int version) const override { return (version >= 11); }
-    virtual int  canUnpack() override { return super::canUnpack(); }  // really 'bool'
+    virtual int  canUnpack() override { return super::canUnpack(); } // bool, except -1: format known, but not packed
 
 protected:
     virtual const int *getCompressionMethods(int method, int level) const override;
@@ -123,7 +123,7 @@ protected:
     virtual void PackLinuxElf32help1(InputFile *f);
     virtual int checkEhdr(Elf32_Ehdr const *ehdr) const;
     virtual bool canPack() override;
-    virtual int  canUnpack() override;  // really 'bool'
+    virtual int  canUnpack() override; // bool, except -1: format known, but not packed
 
     // These ARM routines are essentially common to big/little endian,
     // but the class hierarchy splits after this class.
@@ -266,7 +266,7 @@ protected:
     virtual void PackLinuxElf64help1(InputFile *f);
     virtual int checkEhdr(Elf64_Ehdr const *ehdr) const;
     virtual bool canPack() override;
-    virtual int  canUnpack() override;  // really 'bool'
+    virtual int  canUnpack() override; // bool, except -1: format known, but not packed
 
     virtual void pack1(OutputFile *, Filter &) override;  // generate executable header
     virtual void asl_pack2_Shdrs(OutputFile *);  // AndroidSharedLibrary processes Shdrs
@@ -559,7 +559,7 @@ public:
     virtual const char *getName() const override { return "linux/i386"; }
     virtual const char *getFullName(const options_t *) const override { return "i386-linux.elf"; }
     virtual const int *getFilters() const override;
-    virtual int  canUnpack() override;  // reallly 'bool'
+    virtual int  canUnpack() override; // bool, except -1: format known, but not packed
 
 protected:
     virtual void pack1(OutputFile *, Filter &) override;  // generate executable header
diff --git a/src/p_unix.h b/src/p_unix.h
index 35739bb9..07e669a9 100644
--- a/src/p_unix.h
+++ b/src/p_unix.h
@@ -49,7 +49,7 @@ public:
     virtual void unpack(OutputFile *fo) override;
 
     virtual bool canPack() override;
-    virtual int  canUnpack() override;  // really 'bool'
+    virtual int  canUnpack() override; // bool, except -1: format known, but not packed
     int find_overlay_offset(MemBuffer const &buf);
 
 protected:
diff --git a/src/packer.h b/src/packer.h
index 3ed72373..6e73ed24 100644
--- a/src/packer.h
+++ b/src/packer.h
@@ -155,12 +155,10 @@ protected:
 public:
     // canPack() should throw a cantPackException eplaining why it
     // cannot pack a recognized format.
-    // canUnpack() can return -1 meaning "format recognized, but file
-    // is definitely not packed" (see packmast.cpp).
     virtual bool canPack() = 0;
+    // canUnpack() can return -1 meaning "format recognized, but file
+    // is definitely not packed". See packmast.cpp try_unpack().
     virtual int canUnpack() = 0;
-    virtual int canTest() { return canUnpack(); }
-    virtual int canList() { return canUnpack(); }
 
 protected:
     // main compression drivers