fix(#75): invert core rule check

grammar.go

@@ -460,7 +460,7 @@ func lexABNF(input []byte, path *Path) (any, error) {
 				case "=/":
 					// Block core rules from being used
 					rule := GetRule(rl.Name, nil)
-					if rule == nil {
+					if rule != nil {
 						return nil, &ErrCoreRuleModify{
 							CoreRulename: rl.Name,
 						}

grammar_test.go

@@ -32,6 +32,9 @@ var ruleAbnf []byte
 //go:embed testdata/element.abnf
 var elementAbnf []byte
 
+//go:embed testdata/multi.abnf
+var multiAbnf []byte
+
 //go:embed testdata/abnf.abnf
 var abnfAbnf []byte
 
@@ -92,6 +95,11 @@ var testsParseAbnf = map[string]struct {
 		Validate:  false,
 		ExpectErr: false,
 	},
+	"multi": {
+		Input:     multiAbnf,
+		Validate:  false,
+		ExpectErr: false,
+	},
 	"abnf": {
 		// This test validates we can parse ABNF using ABNF grammar
 		// and the ABNF structural model :)

testdata/multi.abnf

@@ -0,0 +1,2 @@
+a =  "a"
+a =/ "aa"