improve "Constraint value types" paragraph

Author: tisonkun

doc/Language/hashmap.pod6

@@ -10,7 +10,8 @@ The L<Associative> role underlies hashes and maps, as well as other classes such
 as L<MixHash>. It defines the two types that will be used in associative
 classes; by default, you can use anything (literally, since any class that
 subclasses L<Any> can be used) L<as a key|#Non-string_keys_(object_hash),
-although it will be coerced to a string, and any object as value. You can access these types using the C<of> and C<keyof> methods.
+although it will be coerced to a string, and any object as value. You can
+access these types using the C<of> and C<keyof> methods.
 
 By default, any object declared with the C<%> sigil will get the Associative
 role, and will by default behave like a hash, but this role will only provide
@@ -329,20 +330,54 @@ concern itself much.
 =head2 Constraint value types
 
 Place a type object in-between the declarator and the name to constraint the type
-of all values of a C<Hash>. Use a L<subset|/language/typesystem#subset> for
-constraints with a where-clause.
+of all values of a C<Hash>.
 
-    subset Powerful of Int where * > 9000;
-    my Powerful %h{Str};
-    put %h<Goku>   = 9001;
+    my Int %h;
+    put %h<Goku>   = 900;
+    
     try {
-        %h<Vegeta> = 900;
+        %h<Vegeta> = "string";
         CATCH { when X::TypeCheck::Binding { .message.put } }
     }
 
     # OUTPUT:
     # 9001
-    # Type check failed in binding assignval; expected Powerful but got Int (900)
+    # Type check failed in assignment to %h; expected Int but got Str ("string")
+    
+You can do the same by a more readable syntax.
+
+    my %h of Int; # the same as my Int %h
+    
+If you want to constraint the type of all keys of a C<Hash>, add C<{Type}> following
+the name of variable.
+
+    my %h{Int};
+    
+Even put these two constriant together.
+
+    my %h{Int} of Int;
+    put %h{21} = 42;
+    
+    try {
+        %h{0} = "String";
+        CATCH { when X::TypeCheck::Binding { .message.put } }
+    }
+    
+    try {
+        %h<string> = 42;
+        CATCH { when X::TypeCheck::Binding { .message.put } }
+    }
+    
+    try {
+        %h<string> = "String";
+        CATCH { when X::TypeCheck::Binding { .message.put } }
+    }
+    
+    # OUTPUT:
+    # 42
+    # Type check failed in binding to parameter 'assignval'; expected Int but got Str ("String")
+    # Type check failed in binding to parameter 'key'; expected Int but got Str ("string")
+    # Type check failed in binding to parameter 'key'; expected Int but got Str ("string")
 
 =head1 Looping over hash keys and values