Update metafunctions.md (#1015)

Signed-off-by: Neil Henderson <[email protected]>

Author: bluetarpmedia

docs/cpp2/metafunctions.md

@@ -28,7 +28,7 @@ point2d: @value type = {
 
 ## <a id="generating-source"></a>Generating source code at compile time
 
-A metafunction applied to a definition using `@` gets to participate in interpreting the meaning of the definition by inspecting and manipulating the definition's parse tree. For example
+A metafunction applied to a definition using `@` gets to participate in interpreting the meaning of the definition by inspecting and manipulating the definition's parse tree. For example:
 
 ``` cpp title="shape.cpp2: Using @interface @print" hl_lines="1"
 shape: @interface @print type = {
@@ -128,6 +128,17 @@ A `basic_value` type is a regular type: [`copyable`](#copyable), default constru
 
 A `value` (or `weakly_ordered_value` or `partially_ordered_value`) is a `basic_value` that is also [`ordered`](#ordered) (or `weakly_ordered` or `partially_ordered`, respectively).
 
+```mermaid
+graph TD;
+    value---->basic_value;
+    weakly_ordered_value---->basic_value;
+    partially_ordered_value---->basic_value;
+    basic_value-->copyable;
+    value-->ordered;
+    partially_ordered_value-->partially_ordered;
+    weakly_ordered_value-->weakly_ordered;
+```
+
 These metafunctions will emit a compile-time error if:
 
 - any function is protected or virtual
@@ -142,6 +153,7 @@ These metafunctions will emit a compile-time error if:
 >
 > — P0707R4, section 3
 
+
 #### `struct`
 
 A `struct` is a type with only public bases, objects, and functions, with no virtual functions, and with no user-defined constructors (i.e., no invariants) or assignment or destructors.
@@ -187,7 +199,7 @@ Cpp2 has no `interface` feature hardwired into the language, as C# and Java do.
 
 - the type has a copy or move function (the diagnostic message will suggest a virtual `clone` function instead)
 
-- any function has body
+- any function has a body
 
 - any function is nonpublic