feat: Provide grammar producing aterms with signature matching Chap 4

example.

.gitignore

@@ -9,3 +9,4 @@
 /.factorypath
 
 /.polyglot.metaborg.yaml
+*.aterm

editor/Syntax.esv

@@ -8,7 +8,7 @@ imports
 language
 
   table         : target/metaborg/sdf.tbl
-  start symbols : Start
+  start symbols : Prop
 
   line comment  : "//"
   block comment : "/*" * "*/"

prop.spl

@@ -0,0 +1 @@
+(1 -> 0) & 0 = p & 0

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