feat: Provide grammar producing aterms with signature matching Chap 4

example.

syntax/Spoofax-Propositional-Language.sdf3

@@ -6,12 +6,30 @@ imports
 
 context-free start-symbols
   
-  Start
+  Prop
 
 context-free sorts
 
-  Start
+  Prop String
 
 context-free syntax
   
-  Start.Empty = <>
+  Prop.True  = <1>
+  Prop.False = <0>
+  Prop.Atom  = String
+  Prop.Not   = <!<Prop>>
+  Prop.And   = <<Prop> & <Prop>>
+  Prop.Or    = <<Prop> | <Prop>>
+  Prop.Impl  = [[Prop] -> [Prop]]
+  Prop.Eq    = <<Prop> = <Prop>>
+  Prop       = <(<Prop>)> {bracket}
+  
+  String = ID
+
+context-free priorities
+
+  Prop.Not
+  > Prop.And
+  > Prop.Or
+  > Prop.Impl
+  > Prop.Eq