implement grammar and grammarrule

src/main/java/parser/grammar/GrammarRule.java

@@ -0,0 +1,20 @@
+package parser.grammar;
+
+public class GrammarRule {
+
+    private final String leftside;
+    private final String rightside;
+
+    public GrammarRule(String leftside, String... rightside) {
+        this.leftside = leftside;
+        this.rightside = String.join(" ", rightside);
+    }
+
+    public String getLeftside() {
+        return this.leftside;
+    }
+
+    public String getRightside() {
+        return this.rightside;
+    }
+}