add let example for functional interface coercion

Author: puredanger

content/reference/special_forms.adoc

@@ -91,7 +91,10 @@ Evaluates the expressions __expr__s in order and returns the value of the last.
 
 _binding_ => _binding-form_ _init-expr_
 
-Evaluates the expressions __expr__s in a lexical context in which the symbols in the __binding-form__s are bound to their respective __init-expr__s or parts therein. The bindings are sequential, so each _binding_ can see the prior bindings. The __expr__s are contained in an implicit `do`. If a _binding_ symbol is annotated with a metadata tag, the compiler will try to resolve the tag to a class name and presume that type in subsequent references to the _binding_. The simplest _binding-form_ is a symbol, which is bound to the entire _init-expr_:
+Evaluates the expressions __expr__s in a lexical context in which the symbols in the __binding-form__s are bound to their respective __init-expr__s or parts therein. The bindings are sequential, so each _binding_ can see the prior bindings. The __expr__s are contained in an implicit `do`. If a _binding_ symbol is annotated with a metadata tag, the compiler will try to resolve the tag to a class name and presume that type in subsequent references to the _binding_. 
+
+
+The simplest _binding-form_ is a symbol, which is bound to the entire _init-expr_:
 
 [source,clojure]
 ----
@@ -101,6 +104,17 @@ Evaluates the expressions __expr__s in a lexical context in which the symbols in
 -> 1
 ----
 
+If the _binding_ symbol metadata tag is a Java interface annotated as a https://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html[FunctionalInterface], the __init-expr__ will be coerced (if necessary) from the Clojure `IFn` interface to the specified interface:
+
+[source,clojure]
+----
+(let [coll (java.util.ArrayList. (range 10))
+      ^java.util.function.Predicate pred even?]
+  (.removeIf coll pred) ;; mutate coll
+  coll)
+-> [1 3 5 7 9]  
+----
+
 See <<special_forms#binding-forms,Binding Forms>> for more information about binding forms.
 
 *Locals created with `let` are not variables. Once created their values never change!*