diff --git a/INSTALL b/INSTALL
index 32ece23d..e850f7e8 100644
--- a/INSTALL
+++ b/INSTALL
@@ -1,6 +1,6 @@
-INSTALL for Seq66 v. 0.99.9 and above
+INSTALL for Seq66 v. 0.99.12 and above
 Chris Ahlstrom
-2015-09-10 to 2023-09-21
+2015-09-10 to 2023-12-06
 
 Getting Seq66 requires building the code or going to
 "sequencer64-packages/seq66" on GitHub to get an installation package.  The
@@ -88,6 +88,20 @@ Bootstrap Install:
     automake, libtool, and other standard Linux build tools.  This build
     results in the "qseq66" executable.
 
+Clang Compiler:
+
+    If one has the GNU compilers installed, one can still install Clang
+    and use it for the build.
+
+    Assuming the configure script is up-to-date, and either the base clang or
+    a specific version (e.g. clang-16) is installed, then the next command can
+    be used for the base clang, or the second for a specific version:
+
+        $ CC=clang CXX=clang++ ./configure                  - or -
+        $ CC=clang-16 CXX=clang++-16 ./configure
+        $ make &> make.log
+        $ sudo make install
+
 Qt and Command-Line Merged Build:
 
     Per package-manager request, one can build and install both qseq66 and
diff --git a/configure b/configure
index 1f9561f7..918393a8 100755
--- a/configure
+++ b/configure
@@ -3206,7 +3206,197 @@ ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
 ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
 ac_compiler_gnu=$ac_cv_c_compiler_gnu
 if test -n "$ac_tool_prefix"; then
-  for ac_prog in gcc
+  # Extract the first word of "${ac_tool_prefix}gcc", so it can be a program name with args.
+set dummy ${ac_tool_prefix}gcc; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_prog_CC+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if test -n "$CC"; then
+  ac_cv_prog_CC="$CC" # Let the user override the test.
+else
+as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_prog_CC="${ac_tool_prefix}gcc"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+fi
+fi
+CC=$ac_cv_prog_CC
+if test -n "$CC"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $CC" >&5
+$as_echo "$CC" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
+fi
+if test -z "$ac_cv_prog_CC"; then
+  ac_ct_CC=$CC
+  # Extract the first word of "gcc", so it can be a program name with args.
+set dummy gcc; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_prog_ac_ct_CC+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if test -n "$ac_ct_CC"; then
+  ac_cv_prog_ac_ct_CC="$ac_ct_CC" # Let the user override the test.
+else
+as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_prog_ac_ct_CC="gcc"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+fi
+fi
+ac_ct_CC=$ac_cv_prog_ac_ct_CC
+if test -n "$ac_ct_CC"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_ct_CC" >&5
+$as_echo "$ac_ct_CC" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+  if test "x$ac_ct_CC" = x; then
+    CC=""
+  else
+    case $cross_compiling:$ac_tool_warned in
+yes:)
+{ $as_echo "$as_me:${as_lineno-$LINENO}: WARNING: using cross tools not prefixed with host triplet" >&5
+$as_echo "$as_me: WARNING: using cross tools not prefixed with host triplet" >&2;}
+ac_tool_warned=yes ;;
+esac
+    CC=$ac_ct_CC
+  fi
+else
+  CC="$ac_cv_prog_CC"
+fi
+
+if test -z "$CC"; then
+          if test -n "$ac_tool_prefix"; then
+    # Extract the first word of "${ac_tool_prefix}cc", so it can be a program name with args.
+set dummy ${ac_tool_prefix}cc; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_prog_CC+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if test -n "$CC"; then
+  ac_cv_prog_CC="$CC" # Let the user override the test.
+else
+as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    ac_cv_prog_CC="${ac_tool_prefix}cc"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+fi
+fi
+CC=$ac_cv_prog_CC
+if test -n "$CC"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $CC" >&5
+$as_echo "$CC" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
+  fi
+fi
+if test -z "$CC"; then
+  # Extract the first word of "cc", so it can be a program name with args.
+set dummy cc; ac_word=$2
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+$as_echo_n "checking for $ac_word... " >&6; }
+if ${ac_cv_prog_CC+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if test -n "$CC"; then
+  ac_cv_prog_CC="$CC" # Let the user override the test.
+else
+  ac_prog_rejected=no
+as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  test -z "$as_dir" && as_dir=.
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
+    if test "$as_dir/$ac_word$ac_exec_ext" = "/usr/ucb/cc"; then
+       ac_prog_rejected=yes
+       continue
+     fi
+    ac_cv_prog_CC="cc"
+    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+if test $ac_prog_rejected = yes; then
+  # We found a bogon in the path, so make sure we never use it.
+  set dummy $ac_cv_prog_CC
+  shift
+  if test $# != 0; then
+    # We chose a different compiler from the bogus one.
+    # However, it has the same basename, so the bogon will be chosen
+    # first if we set CC to just the basename; use the full file name.
+    shift
+    ac_cv_prog_CC="$as_dir/$ac_word${1+' '}$@"
+  fi
+fi
+fi
+fi
+CC=$ac_cv_prog_CC
+if test -n "$CC"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $CC" >&5
+$as_echo "$CC" >&6; }
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+
+fi
+if test -z "$CC"; then
+  if test -n "$ac_tool_prefix"; then
+  for ac_prog in cl.exe
   do
     # Extract the first word of "$ac_tool_prefix$ac_prog", so it can be a program name with args.
 set dummy $ac_tool_prefix$ac_prog; ac_word=$2
@@ -3250,7 +3440,7 @@ fi
 fi
 if test -z "$CC"; then
   ac_ct_CC=$CC
-  for ac_prog in gcc
+  for ac_prog in cl.exe
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
 set dummy $ac_prog; ac_word=$2
@@ -3305,6 +3495,8 @@ esac
   fi
 fi
 
+fi
+
 
 test -z "$CC" && { { $as_echo "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
 $as_echo "$as_me: error: in \`$ac_pwd':" >&2;}
@@ -4057,7 +4249,7 @@ if test -z "$CXX"; then
     CXX=$CCC
   else
     if test -n "$ac_tool_prefix"; then
-  for ac_prog in g++
+  for ac_prog in g++ c++ gpp aCC CC cxx cc++ cl.exe FCC KCC RCC xlC_r xlC
   do
     # Extract the first word of "$ac_tool_prefix$ac_prog", so it can be a program name with args.
 set dummy $ac_tool_prefix$ac_prog; ac_word=$2
@@ -4101,7 +4293,7 @@ fi
 fi
 if test -z "$CXX"; then
   ac_ct_CXX=$CXX
-  for ac_prog in g++
+  for ac_prog in g++ c++ gpp aCC CC cxx cc++ cl.exe FCC KCC RCC xlC_r xlC
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
 set dummy $ac_prog; ac_word=$2
diff --git a/configure.ac b/configure.ac
index 5dcfa090..728af475 100644
--- a/configure.ac
+++ b/configure.ac
@@ -37,9 +37,12 @@ AX_PREFIX_CONFIG_H([include/seq66-config.h], [seq66])
 
 dnl gcc/g++ configuration.
 
-AC_PROG_CC([gcc])
-AC_PROG_CXX([g++])
-AC_PROG_CXXCPP([cpp])
+dnl AC_PROG_CC([gcc])
+dnl AC_PROG_CXX([g++])
+dnl AC_PROG_CXXCPP([cpp])
+AC_PROG_CC
+AC_PROG_CXX
+AC_PROG_CXXCPP
 AC_PROG_INSTALL
 AC_PROG_LN_S
 AC_PROG_MKDIR_P
diff --git a/libseq66/include/play/mutegroup.hpp b/libseq66/include/play/mutegroup.hpp
index db5c45a9..12c128de 100644
--- a/libseq66/include/play/mutegroup.hpp
+++ b/libseq66/include/play/mutegroup.hpp
@@ -29,7 +29,7 @@
  * \library       seq66 application
  * \author        Chris Ahlstrom
  * \date          2018-12-01
- * \updates       2022-06-28
+ * \updates       2023-12-06
  * \license       GNU GPLv2 or above
  *
  */
@@ -99,9 +99,13 @@ class mutegroup
      *  of row x column.  It is important to note the the size of the group is
      *  constant throughout its lifetime (and the lifetime of the
      *  application).
+     *
+     *  For issue #124, we have removed the const from some member declarations
+     *  so that the default constructor etc. are not deleted. Thanks to clang
+     *  for uncovering that.
      */
 
-    const int m_group_size;
+    /* const */ int m_group_size;
 
     /**
      *  Holds a set of boolean values in a 1-D vector, but can be virtually
@@ -119,7 +123,7 @@ class mutegroup
      *  mute-group.
      */
 
-    const int m_rows;
+    /* const */ int m_rows;
 
     /**
      *  Indicates the number of virtual columns in a screen-set (bank), which
@@ -129,7 +133,7 @@ class mutegroup
      *  per set or mute-group.
      */
 
-    const int m_columns;
+    /* const */ int m_columns;
 
     /**
      *  Experimental option to swap rows and columns.  See the function
@@ -153,7 +157,7 @@ class mutegroup
      *  saves a calculation.
      */
 
-    const int m_group_offset;
+    /* const */ int m_group_offset;
 
 public:
 
diff --git a/libseq66/include/play/notemapper.hpp b/libseq66/include/play/notemapper.hpp
index b5dbe537..76c13b7c 100644
--- a/libseq66/include/play/notemapper.hpp
+++ b/libseq66/include/play/notemapper.hpp
@@ -30,7 +30,7 @@
  * \library       libmidipp
  * \author        Chris Ahlstrom
  * \date          2014-04-24
- * \updates       2023-09-01
+ * \updates       2023-12-06
  * \version       $Revision$
  * \license       GNU GPL
  *
@@ -115,9 +115,12 @@ class notemapper final : public basesettings
         /**
          *  Indicates if this is a reversed pair.  This boolean is needed to
          *  determine whether the dev-note or the gm-note is the key value.
+         *
+         *  For issue #124, clang deletes the assignment operator, so we
+         *  get rid of the consts.
          */
 
-        const bool m_is_reverse;
+        /* const */ bool m_is_reverse;
 
         /**
          *  The incoming note number from a non-GM compliant device.  This
@@ -125,7 +128,7 @@ class notemapper final : public basesettings
          *  array.
          */
 
-        const int m_dev_value;
+        /* const */ int m_dev_value;
 
         /**
          *    The integer value to which the incoming (key) value is to be
@@ -133,13 +136,13 @@ class notemapper final : public basesettings
          *    device.
          */
 
-        const int m_gm_value;
+        /* const */ int m_gm_value;
 
         /**
          *  The name of the key as represented by the non-GM device.
          */
 
-        const std::string m_dev_name;
+        /* const */ std::string m_dev_name;
 
         /**
          *    The name of the GM drum note or patch that is replacing the
@@ -148,7 +151,7 @@ class notemapper final : public basesettings
          *    device's sound.
          */
 
-        const std::string m_gm_name;
+        /* const */ std::string m_gm_name;
 
         /**
          *    The number of times this particular mapping was performed in the
diff --git a/libseq66/include/play/portslist.hpp b/libseq66/include/play/portslist.hpp
index 055edd6d..79651f1f 100644
--- a/libseq66/include/play/portslist.hpp
+++ b/libseq66/include/play/portslist.hpp
@@ -85,7 +85,7 @@ class portslist
         on                              /**< Activate the list.             */
     };
 
-protected:
+public:
 
     /**
      *  Provides a port name and the input or output values.  Note that the
@@ -109,6 +109,8 @@ class portslist
         int io_port_number;         /**< The system port number.            */
     };
 
+protected:
+
     /**
      *  The container type for io information.  Replaces std::vector<io>.
      */
diff --git a/libseq66/src/cfg/mutegroupsfile.cpp b/libseq66/src/cfg/mutegroupsfile.cpp
index 9540d702..07fac5cc 100644
--- a/libseq66/src/cfg/mutegroupsfile.cpp
+++ b/libseq66/src/cfg/mutegroupsfile.cpp
@@ -25,7 +25,7 @@
  * \library       seq66 application
  * \author        Seq24 team; modifications by Chris Ahlstrom
  * \date          2018-11-13
- * \updates       2023-10-19
+ * \updates       2023-12-06
  * \license       GNU GPLv2 or above
  *
  */
@@ -342,13 +342,13 @@ mutegroupsfile::write ()
 }
 
 /**
- *  The default long format for writing mute groups.
+ *  The default long format for writing mute groups. No longer needed.
+ *
+ *      static const char * const sg_scanf_fmt_1 =
+ *        "%d [ %d %d %d %d %d %d %d %d ] [ %d %d %d %d %d %d %d %d ] "
+ *        " [ %d %d %d %d %d %d %d %d ] [ %d %d %d %d %d %d %d %d ]" ;
  */
 
-static const char * const sg_scanf_fmt_1 =
-    "%d [ %d %d %d %d %d %d %d %d ] [ %d %d %d %d %d %d %d %d ] "
-      " [ %d %d %d %d %d %d %d %d ] [ %d %d %d %d %d %d %d %d ]";
-
 /**
  *  Writes the [mute-group] section to the given file stream.  This can also
  *  be called by the rcfile object to just dump the data into that file.
diff --git a/libseq66/src/cfg/usrfile.cpp b/libseq66/src/cfg/usrfile.cpp
index 799e6665..8ca01beb 100644
--- a/libseq66/src/cfg/usrfile.cpp
+++ b/libseq66/src/cfg/usrfile.cpp
@@ -26,7 +26,7 @@
  * \library       seq66 application
  * \author        Chris Ahlstrom
  * \date          2018-11-23
- * \updates       2023-11-05
+ * \updates       2023-12-06
  * \license       GNU GPLv2 or above
  *
  *  Note that the parse function has some code that is not yet enabled.
@@ -188,7 +188,7 @@ usrfile::parse ()
                 msgprintf
                 (
                     msglevel::error, "Error adding %s (line = '%s')",
-                    label, scanline()
+                    label.c_str(), scanline()           /* see issue #124   */
                 );
             }
         }
@@ -249,10 +249,15 @@ usrfile::parse ()
         }
         else
         {
+            /*
+             * clang says "error: member reference base type 'const char *'
+             * is not a structure or union." See issue #124.
+             */
+
             msgprintf
             (
                 msglevel::error, "Error adding %s (line = '%s')",
-                label, scanline()
+                label.c_str(), scanline()
             );
         }
     }
diff --git a/libseq66/src/cfg/usrsettings.cpp b/libseq66/src/cfg/usrsettings.cpp
index 6c55a8a4..ca9ef82a 100644
--- a/libseq66/src/cfg/usrsettings.cpp
+++ b/libseq66/src/cfg/usrsettings.cpp
@@ -25,7 +25,7 @@
  * \library       seq66 application
  * \author        Chris Ahlstrom
  * \date          2015-09-23
- * \updates       2023-11-25
+ * \updates       2023-12-06
  * \license       GNU GPLv2 or above
  *
  *  Note that this module also sets the remaining legacy global variables, so
@@ -237,6 +237,14 @@ static const int c_default_redraw_ms = 40;
 #endif
 
 /**
+ *  Constants for the main window, etc. The c_seqchars_x and c_seqchars_y
+ *  constants help define the "seqarea" size.  These look like the number
+ *  of characters per line and the number of lines of characters, in a
+ *  pattern/sequence box.
+ *
+ *      static const int c_seqchars_x = 15;
+ *      static const int c_seqchars_y =  5;
+ *
  *  These control sizes.  We'll try changing them and see what happens.
  *  Increasing these value spreads out the pattern grids a little bit and
  *  makes the Patterns panel slightly bigger.  Seems like it would be
@@ -247,30 +255,20 @@ static const int c_default_redraw_ms = 40;
  *  the character width (x) and height (y) in pixels.  Thus, these values
  *  would be dependent on the font chosen.  But that, currently, is
  *  hard-wired.
- */
-
-static const int c_text_x =  6;            /* doesn't include inner padding */
-static const int c_text_y = 12;            /* does include inner padding    */
-
-/**
- *  Constants for the main window, etc. The c_seqchars_x and c_seqchars_y
- *  constants help define the "seqarea" size.  These look like the number
- *  of characters per line and the number of lines of characters, in a
- *  pattern/sequence box.
- */
-
-static const int c_seqchars_x = 15;
-static const int c_seqchars_y =  5;
-
-/**
+ *
+ *      static const int c_text_x =  6;    // doesn't include inner padding
+ *      static const int c_text_y = 12;    // does include inner padding
+ *
  *  The c_seqarea_x and c_seqarea_y constants are derived from the width
  *  and heights of the default character set, and the number of characters
  *  in width, and the number of lines, in a pattern/sequence box.
+ *
+ *  Clang reveals they are not used.
+ *
+ *      static const int c_seqarea_x = c_text_x * c_seqchars_x;
+ *      static const int c_seqarea_y = c_text_y * c_seqchars_y;
  */
 
-static const int c_seqarea_x = c_text_x * c_seqchars_x;
-static const int c_seqarea_y = c_text_y * c_seqchars_y;
-
 /**
  *  These control sizes.  We'll try changing them and see what happens.
  *  Increasing these value spreads out the pattern grids a little bit and
diff --git a/libseq66/src/ctrl/keymap.cpp b/libseq66/src/ctrl/keymap.cpp
index 996c153f..5cb2d059 100644
--- a/libseq66/src/ctrl/keymap.cpp
+++ b/libseq66/src/ctrl/keymap.cpp
@@ -25,7 +25,7 @@
  * \library       seq66 application
  * \author        Chris Ahlstrom
  * \date          2018-11-12
- * \updates       2023-09-02
+ * \updates       2023-12-06
  * \license       GNU GPLv2 or above
  */
 
@@ -127,10 +127,11 @@ arrow_down ()
 /**
  *  Indicates the start of section where expansion/foreign characters can be
  *  placed.  Covers the ordinals from 0xe0 to 0xfe (31 characters).
+ *  Not yet used, as revealed by clang-12.
+ *
+ *      static const int s_expansion_start = 0xe0;
  */
 
-static const int s_expansion_start = 0xe0;
-
 /**
  *  Provides a data type for key name/value pairs.
  */
diff --git a/libseq66/src/seq66_features.cpp b/libseq66/src/seq66_features.cpp
index 597453bb..a58e008b 100644
--- a/libseq66/src/seq66_features.cpp
+++ b/libseq66/src/seq66_features.cpp
@@ -25,7 +25,7 @@
  * \library       seq66 application
  * \author        Chris Ahlstrom
  * \date          2017-03-12
- * \updates       2023-10-28
+ * \updates       2023-12-06
  * \license       GNU GPLv2 or above
  *
  *  The first part of this file defines a couple of global structure
@@ -547,9 +547,13 @@ seq_build_details ()
     result
         << "Built " << __DATE__ << " " << __TIME__ "\n"
         << "C++ version " << std::to_string(__cplusplus) << "\n"
+#if defined __clang__
+        << "Clang C++ " << __clang_version__ << "\n"
+#else
 #if defined SEQ66_PLATFORM_GNU
         << "GNU C++ " << __GNUC__ << "." << __GNUC_MINOR__
         << "." << __GNUC_PATCHLEVEL__ << "\n"
+#endif
 #endif
         << "Executable: " << seq_app_name() << " (" << seq_app_path() << ")\n"
         << "Interface: " << seq_app_type() << "\n"