Traits: traits that require other traits

A trait may require another of every implementor, with a where clause
in the trait. The section shows a function that asks for one trait and
uses the other, and notes that Ord now requires Eq.

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Co-Authored-By: eca-agent <git@eca.dev>

_01.Language-basics.sz

@@ -2152,6 +2152,26 @@ A blanket implementation is one for a bare type variable. It applies to every ty
 (weight "hello")   ;; 5
 }|
 
+@subsubsection{Traits that require other traits}
+
+A trait may say that every type implementing it implements another trait too, with a @code{where} in the trait itself. Something that greets must have a name:
+
+@codeblock[#:lang "bjolang"]|{
+(def/trait (Named %a)
+  (: name-of (-> %a string)))
+
+(def/trait (Greet %a)
+  (where (Named %a))
+  (: greeting (-> %a string)))
+
+(: introduce (-> %a string) (where (Greet %a)))
+(defun (introduce x) (str (name-of x) " says " (greeting x)))
+}|
+
+@code{introduce} asks only for @code{Greet}, and may still call @code{name-of}: a type that can greet is known to have a name. An implementation of @code{Greet} for a type with no @code{Named} is a compile error at the implementation.
+
+The prelude's @code{Ord} requires @code{Eq} this way, so a function with @code{(where (Ord %a))} can use @code{=} as well as @code{compare}.
+
 @subsubsection{Eq, Ord and ->str}
 
 Three traits from the prelude are worth knowing by heart, since the rest of the library leans on them.