muscles: fix handling of the "@'" escape

When we use `b4_` or `m4_` somewhere in the input, it is escaped as
`b4@'_`/`m4@'_` so that the warning about unexpanded b4_foo/m4_foo
macros does not fire.

But in the case of muscles, the `@'` escape was not recognized, and an
assertion was triggered.

Reported by Han Zheng.
<#91>

* src/muscle-tab.c (COMMON_DECODE): Handle `@'`.
* tests/skeletons.at (Suspicious sequences): Check that case.

src/muscle-tab.c

@@ -294,10 +294,11 @@ muscle_location_grow (char const *key, location loc)
   case '@':                                                     \
     switch (*++(Value))                                         \
       {                                                         \
-        case '@': obstack_sgrow (&muscle_obstack, "@" ); break; \
-        case '{': obstack_sgrow (&muscle_obstack, "[" ); break; \
-        case '}': obstack_sgrow (&muscle_obstack, "]" ); break; \
-        default: aver (false); break;                           \
+      case '\'': /* Ignore "@'" */ break;                       \
+      case '@': obstack_sgrow (&muscle_obstack, "@" ); break;   \
+      case '{': obstack_sgrow (&muscle_obstack, "[" ); break;   \
+      case '}': obstack_sgrow (&muscle_obstack, "]" ); break;   \
+      default: aver (false); break;                             \
       }                                                         \
     break;                                                      \
   default:                                                      \

tests/skeletons.at

@@ -369,5 +369,20 @@ AT_BISON_CHECK([[input1.y]], [], [],
 input1.tab.c:13: warning: suspicious sequence in the output: m4@&t@_poison [-Wother]
 ]])
 
+# Regression test for <https://github.com/akimd/bison/issues/91>.
+AT_DATA([[input2.y]],
+[[%define parse.trace {m4@&t@_foo}
+%debug
+%%
+exp:
+]])
+
+AT_BISON_CHECK([[input2.y]], [1], [],
+[[input2.y:1.1-28: warning: %define variable 'parse.trace' requires keyword values [-Wdeprecated]
+input2.y:1.1-28: error: invalid value for %define Boolean variable 'parse.trace'
+input2.y:2.1-6: error: %define variable 'parse.trace' redefined
+input2.y:1.1-28: note: previous definition
+input2.y: warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]
+]])
 
 AT_CLEANUP