'a' as an operand is ignored.

Based on testing with Merlin-16+ 4.12, this seems to apply in all cases

inc a -> equivalent to inc
lda a -> equivalent to lda (bad address mode error)

if you do have an 'a' label, you need to specify the address mode, eg

lda |a, lda >a, lda <a

src/asm/asm.1.s

@@ -2418,7 +2418,11 @@ addmode      php
              beq   :skipq
              cmp   #$22 ; "
              beq   :skipq
+             and   #$5f
+             cmp   #'A'
+             beq   :a
              jmp   :index
+
 :force8      lda   #amforce8
              tsb   myvalue
              jmp   :index
@@ -2435,6 +2439,14 @@ addmode      php
              tsb   myvalue
              jmp   :index
 
+*  'a' (no modifier) is ignored
+:a           iny
+             lda [lineptr],y
+             cmp #' '+1
+             jlt :zero
+             dey
+             jmp :index
+
 * skip over quoted operand component.
 :skipq       sta   :q
 :skipq1      iny

testdata/3015-amacc.S

@@ -0,0 +1,10 @@
+*
+* explicit a operand (inc a, etc) is dropped.
+*
+
+	inc a
+	dec a
+	lsr a
+	asl a
+	rol a
+	ror a