Amend grammar for rulings 18 and 26

src/main/antlr4/MPL.g4

@@ -166,9 +166,11 @@ primary
     | list
     ;
 
+// Ruling 26: λ is reserved — never an identifier. Every other Greek
+// letter remains one (Fatima wants π).
 greekVar
     : ALPHA | BETA | GAMMA | DELTA | EPSILON | ZETA | ETA | THETA
-    | IOTA | KAPPA | LAMBDA_VAR | MU | NU | XI | OMICRON | PI
+    | IOTA | KAPPA | MU | NU | XI | OMICRON | PI
     | RHO | SIGMA | TAU | UPSILON | PHI | CHI | PSI | OMEGA
     ;
 
@@ -177,8 +179,9 @@ typeSymbol
     ;
 
 // λx: body   λx,y: body   λx∈ℝ: body — parameters are a bare pattern list.
+// Ruling 18: nullary functions exist — `λ: e` (bare colon) is admitted.
 lambda
-    : LAMBDA_VAR pattern (IN condExpr)? COLON expr
+    : LAMBDA_VAR (pattern (IN condExpr)?)? COLON expr
     ;
 
 forall
@@ -409,9 +412,9 @@ RAWSTRING
     : '"""' .*? '"""'
     ;
 
+// Ruling 12: string escapes are exactly \n \t \" \\ — nothing else.
 fragment ESC
-    : '\\' [\\nrt0"]
-    | '\\u{' [0-9a-fA-F]+ '}'
+    : '\\' [\\nt"]
     ;
 
 // Comments

src/test/java/com/mpl/test/ParserTest.java

@@ -86,6 +86,7 @@ public class ParserTest extends MPLTestBase {
         assertParses("pi ≜ 3.14159;");
         assertParses("id ≜ λx: x;");
         assertParses("π ≜ 3.14159;");          // greek letter on the left
+        assertParses("f ≜ λ: 1;");             // ruling 18: nullary λ, bare colon
     }
     
     @Test
@@ -237,9 +238,13 @@ public class ParserTest extends MPLTestBase {
         assertDoesNotParse("x ++ y;");
         assertDoesNotParse("a ** b;");
         
-        // Invalid lambda syntax
-        assertDoesNotParse("λ: x;");
+        // Invalid lambda syntax (λ: x became VALID under ruling 18)
         assertDoesNotParse("λx y: x + y;");
+        // Ruling 25: the parenthesized parameter spelling stays rejected.
+        assertDoesNotParse("λ(a, b): a;");
+        // Ruling 26: λ is reserved — never an identifier.
+        assertDoesNotParse("λ ≜ 3;");
+        assertDoesNotParse("{λ};");
 
         // Juxtaposition application was removed — calls need parentheses
         assertDoesNotParse("f x;");