diff --git a/antlr/pdp7asm.g4 b/antlr/pdp7asm.g4
index 6279280..7954709 100644
--- a/antlr/pdp7asm.g4
+++ b/antlr/pdp7asm.g4
@@ -76,18 +76,22 @@ atom
    | DECIMAL
    | DECIMAL_MINUS
    | OCTAL
-   | OCTAL_LITERAL
-   | d
+   | signeddecimal
+   | signedoctal
    ;
 
 string
-    : STRING '>'?
+    : STRING OCTAL_LITERAL* '>'?
     ;
 
-d
+signeddecimal
    : '-'? DECIMAL_LITERAL
    ;
 
+signedoctal
+     : '-'? OCTAL_LITERAL
+     ;
+
 eol
    : EOL
    ;