#2196 Enhance BNF with the ability to specify labels in rule bodies

platform-eql-grammar/src/main/java/fielden/platform/eql/fling/BNF.java

@@ -284,11 +284,23 @@ private void add(final Token t) {
             tokens.add(t);
         }
 
+        private void add(final LabeledTempSymbol symbol) {
+            add(symbol.symbol().normalize());
+        }
+
+        private void add(final Symbol symbol) {
+            switch (symbol) {
+                case Variable v -> add(v);
+                case Token tok -> add(tok);
+                case LabeledTempSymbol s -> add(s);
+                default -> throw new IllegalStateException("Unexpected value: " + symbol);
+            }
+        }
+
         private void add(final Component component) {
             switch (component) {
                 case Quantifier q -> add(q);
-                case Token tok -> add(tok);
-                case Variable v -> add(v);
+                case Symbol sym -> add(sym);
                 default -> throw new IllegalStateException("Unexpected value: " + component);
             }
         }

platform-eql-grammar/src/main/java/fielden/platform/eql/fling/LabeledTempSymbol.java

@@ -0,0 +1,83 @@
+package fielden.platform.eql.fling;
+
+import il.ac.technion.cs.fling.internal.grammar.rules.*;
+
+import java.util.Objects;
+
+public abstract sealed class LabeledTempSymbol<T extends TempSymbol> implements TempSymbol {
+    protected final String label;
+    protected final T symbol;
+
+    LabeledTempSymbol(String label, T symbol) {
+        this.label = label;
+        this.symbol = symbol;
+    }
+
+    @Override
+    public String name() {
+        return symbol.name();
+    }
+
+    public String label() {
+        return label;
+    }
+
+    public T symbol() {
+        return symbol;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (obj == this)
+            return true;
+        if (obj == null || obj.getClass() != this.getClass())
+            return false;
+        var that = (LabeledTempSymbol) obj;
+        return Objects.equals(this.label, that.label) &&
+                Objects.equals(this.symbol, that.symbol);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(label, symbol);
+    }
+
+    @Override
+    public String toString() {
+        return "%s=%s".formatted(label, symbol);
+    }
+
+    // if the label support needs to be extended to other types (e.g., Token)
+    // create a corresponding class and a static method in a manner similar to LabeledVariable
+
+    public static LabeledVariable label(String label, Variable variable) {
+        return new LabeledVariable(label, variable);
+    }
+
+    public static final class LabeledVariable extends LabeledTempSymbol<Variable> implements Variable {
+        public LabeledVariable(final String label, final Variable symbol) {
+            super(label, symbol);
+        }
+
+        @Override
+        public Symbol normalize() {
+            return this;
+        }
+    }
+
+    public static LabeledTerminal label(String label, Terminal variable) {
+        return new LabeledTerminal(label, variable);
+    }
+
+    public static final class LabeledTerminal extends LabeledTempSymbol<Terminal> implements Terminal {
+        public LabeledTerminal(final String label, final Terminal symbol) {
+            super(label, symbol);
+        }
+
+        @Override
+        public Token normalize() {
+            return new Token(this);
+        }
+    }
+
+}