Document the fact that we don't implement ,, with an arg.

The arg could be a constant string or a glob.  The semantics are quite
weird.

core/libstr.py

@@ -177,21 +177,28 @@ def DoUnarySuffixOp(s, op, arg):
 
     elif op.op_id in (Id.VOp1_Percent, Id.VOp1_DPercent):  # const suffix
       if s.endswith(arg):
-        # Mutate it so we preserve the flags.
         return s[:-len(arg)]
       else:
         return s
 
     elif op.op_id == Id.VOp1_Comma:  # Only lowercase the first letter
+      if arg != '':
+        raise NotImplementedError("%s can't have an argument" % op.op_id)
       return s[0].lower() + s[1:]
 
     elif op.op_id == Id.VOp1_DComma:
+      if arg != '':
+        raise NotImplementedError("%s can't have an argument" % op.op_id)
       return s.lower()
 
     elif op.op_id == Id.VOp1_Caret:  # Only uppercase the first letter
+      if arg != '':
+        raise NotImplementedError("%s can't have an argument" % op.op_id)
       return s[0].upper() + s[1:]
 
     elif op.op_id == Id.VOp1_DCaret:
+      if arg != '':
+        raise NotImplementedError("%s can't have an argument" % op.op_id)
       return s.upper()
 
     else:  # e.g. ^ ^^ , ,,
@@ -251,6 +258,9 @@ def DoUnarySuffixOp(s, op, arg):
     else:
       return s
 
+  else:
+    raise NotImplementedError("Can't use %s with pattern" % op.op_id)
+
 
 def _AllMatchPositions(s, regex):
   """Returns a list of all (start, end) match positions of the regex against s.

spec/var-op-other.test.sh

@@ -466,3 +466,26 @@ ABC DEF
 ## N-I dash status: 2
 ## N-I mksh/zsh status: 1
 
+#### Lower Case with constant string (VERY WEIRD)
+x='AAA ABC DEF'
+echo ${x,A}
+echo ${x,,A}  # replaces every A only?
+## STDOUT:
+aAA ABC DEF
+aaa aBC DEF
+## END
+## N-I dash/mksh/zsh stdout-json: ""
+## N-I dash status: 2
+## N-I mksh/zsh status: 1
+
+#### Lower Case glob
+x='ABC DEF'
+echo ${x,[d-f]}
+echo ${x,,[d-f]}  # This seems buggy, it doesn't include F?
+## STDOUT:
+ABC DEF
+ABC deF
+## END
+## N-I dash/mksh/zsh stdout-json: ""
+## N-I dash status: 2
+## N-I mksh/zsh status: 1

test/spec.sh

@@ -439,7 +439,7 @@ var-op-test() {
 }
 
 var-op-other() {
-  sh-spec spec/var-op-other.test.sh \
+  sh-spec spec/var-op-other.test.sh --osh-failures-allowed 2 \
     ${REF_SHELLS[@]} $ZSH $OSH_LIST "$@"
 }