Correct reference to definition of flatten

docs/language/dartLangSpec.tex

 \documentclass{article}
 \usepackage{epsfig}
 \usepackage{color}
 \usepackage{dart}
 \usepackage{bnf}
 \usepackage{hyperref}
 \usepackage{lmodern}
 \newcommand{\code}[1]{{\sf #1}}
 \title{Dart Programming Language  Specification \\
 {\large Version 1.9}}
@@ skipping 985 matching lines @@
 
 \LMHash{}
 It is a compile-time error if the arity of the user-declared operator  \code{ \~{}} is not 0. 
 
 \LMHash{}
 It is a compile-time error to declare an optional parameter in an operator.
 
 \LMHash{}
 It is a static warning if the return type of the user-declared operator \code{[]=} is explicitly declared and not \VOID{}.
 
+% add rationale: return in []= methods will have no effect, a the expression always returns its second argument (the RHS of the assignment, for consistency with assignment in general). So it's best to enforce this by declaring the method to be void, even though the expression that uses it returns an object with the type of the RHS, as described in \ref{assignment}.
+
 
 \subsection{Getters} 
 \LMLabel{getters}
 
 \LMHash{}
 Getters are functions (\ref{functions})  that are used to retrieve the values of object properties.
 
 \begin{grammar}
 {\bf getterSignature:}
        returnType? \GET{} identifier 
@@ skipping 2067 matching lines @@
 
 $(T_1$ $a_1, \ldots, T_n$ $a_n, [T_{n+1}$ $x_{n+1} = d_1, \ldots,  T_{n+k}$ $x_{n+k} = d_k]) => e$ 
 is 
 
 $(T_1 \ldots, T_n, [T_{n+1}$ $x_{n+1}, \ldots, T_{n+k}$ $x_{n+k}]) \rightarrow T_0$, where $T_0$ is the static type of $e$. 
 
 \LMHash{}
 The static type of a function literal of the form 
 
 $(T_1$ $a_1, \ldots, T_n$ $a_n, [T_{n+1}$ $x_{n+1} = d_1, \ldots,  T_{n+k}$ $x_{n+k} = d_k])$ \ASYNC{} $=> e$
-is $(T_1 \ldots, T_n, [T_{n+1}$ $x_{n+1}, \ldots, T_{n+k}$ $x_{n+k}]) \rightarrow Future<T_0>$, where $T_0$ is the static type of $e$. 
+is $(T_1 \ldots, T_n, [T_{n+1}$ $x_{n+1}, \ldots, T_{n+k}$ $x_{n+k}]) \rightarrow Future<flatten(T_0)>$, where $T_0$ is the static type of $e$ and  $flatten(T) = flatten(S)$ if $T = Future<S>$, and $T$ otherwise. 
+
+\rationale{
+We collapse multiple layers of futures into one. If $e$ evaluates to a future $f$, the future will not invoke its \code{then()} callback until f completes to a non-future value, and so the result of an await is never a future, and the result of an async function will never have type \code{Future$<X>$} where $X$ itself is an invocation of \code{Future}. 
+}
+
 
 \LMHash{}
 The static type of a function literal of the form 
 
 $(T_1$ $a_1, \ldots, T_n$ $a_n, \{T_{n+1}$ $x_{n+1} : d_1, \ldots,  T_{n+k}$ $x_{n+k} : d_k\}) => e$ 
 is 
 
 $(T_1 \ldots, T_n, \{T_{n+1}$ $x_{n+1}, \ldots, T_{n+k}$ $x_{n+k}\}) \rightarrow T_0$, where $T_0$ is the static type of $e$. 
 
 \LMHash{}
 The static type of a function literal of the form 
 
 $(T_1$ $a_1, \ldots, T_n$ $a_n, \{T_{n+1}$ $x_{n+1} : d_1, \ldots,  T_{n+k}$ $x_{n+k} : d_k\})$ \ASYNC{}  $=> e$
 
-is $(T_1 \ldots, T_n, \{T_{n+1}$ $x_{n+1}, \ldots, T_{n+k}$ $x_{n+k}\}) \rightarrow Future<T_0>$, where $T_0$ is the static type of $e$. 
+is $(T_1 \ldots, T_n, \{T_{n+1}$ $x_{n+1}, \ldots, T_{n+k}$ $x_{n+k}\}) \rightarrow Future<flatten(T_0)>$, where $T_0$ is the static type of $e$. 
 
 \LMHash{}
 The static type of a function literal of the form  
 
 $(T_1$ $a_1, \ldots, T_n$ $a_n, [T_{n+1}$ $x_{n+1} = d_1, \ldots,  T_{n+k}$ $x_{n+k}= d_k])\{s\}$ 
 
 is $(T_1 \ldots, T_n, [T_{n+1}$ $x_{n+1}, \ldots, T_{n+k}$ $x_{n+k}]) \rightarrow  \DYNAMIC{}$.
 
 \LMHash{}
 The static type of a function literal of the form  
@@ skipping 653 matching lines @@
 }
 
 \LMHash{}
 If  $n < h$, or $n > m$, the method lookup has failed. Furthermore, each $x_i, n+1 \le i \le n+k$,  must have a corresponding named parameter in the set $\{p_{m+1}, \ldots, p_{h+l}\}$ or the method lookup also fails.  If $v_o$ is an instance of \code{Type} but $o$ is not a constant type literal, then if $m$ is a method that forwards (\ref{functionDeclarations}) to a static method, method lookup fails. Otherwise method lookup has succeeded.
 
 \LMHash{}
 If the method lookup succeeded, the body of $f$ is executed with respect to the bindings that resulted from the evaluation of the argument list, and with \THIS{} bound to $v_o$. The value of $i$ is the value returned after $f$ is executed.
 
 \LMHash{}
 If the method lookup has failed, then let $g$ be the result of looking up getter (\ref{getterAndSetterLookup}) $m$ in $v_o$ with respect to $L$. 
-f $v_o$ is an instance of \code{Type} but $o$ is not a constant type literal, then if $g$ is a getter that forwards to a static getter, getter lookup fails.
+If $v_o$ is an instance of \code{Type} but $o$ is not a constant type literal, then if $g$ is a getter that forwards to a static getter, getter lookup fails.
 If the getter lookup succeeded, let $v_g$ be the value of the getter invocation $o.m$. Then the value of $i$ is the result of invoking 
 the static method \code{Function.apply()} with arguments $v.g, [o_1, \ldots , o_n], \{x_{n+1}: o_{n+1}, \ldots , x_{n+k}: o_{n+k}\}$.
 
 \LMHash{}
 If  getter lookup has also failed, then a new instance $im$  of the predefined class  \code{Invocation}  is created, such that :
 \begin{itemize}
 \item  \code{im.isMethod} evaluates to \code{\TRUE{}}.
 \item  \code{im.memberName} evaluates to \code{'m'}.
 \item \code{im.positionalArguments} evaluates to an immutable list with the same values as  \code{[$o_1, \ldots, o_n$]}.
 \item \code{im.namedArguments} evaluates to an immutable map with the same keys and values as \code{\{$x_{n+1}: o_{n+1}, \ldots, x_{n+k} : o_{n+k}$\}}.
@@ skipping 796 matching lines @@
 
 \rationale{
 An await expression has no meaning in a synchronous function. If such a function were to suspend waiting for a future, it would no longer be synchronous.
 }
 
 \commentary{
 It is not a static warning if the type of $e$ is not a subtype of \code{Future}. Tools may choose to give a hint in such cases.
 }
 
 \LMHash{}
-Let $flatten(T) = flatten(S)$ if $T = Future<S>$, and $T$ otherwise. The static type of $a$ is $flatten(T)$ where $T$ is the static type of $e$.
-
-\rationale{
-We collapse multiple layers of futures into one. If $e$ evaluates to a future $f$, the future will not invoke its \code{then()} callback until f completes to a non-future value, and so the result of an await is never a future, and the result of an async function will never have type \code{Future$<X>$} where $X$ itself is an invocation of \code{Future}. 
-}
-
+The static type of $a$ is $flatten(T)$ (the $flatten$ function is defined in section \ref{functionExpressions}) where $T$ is the static type of $e$.
 
 
      
 \subsection{ Postfix Expressions}
 \LMLabel{postfixExpressions}
 
 \LMHash{}
 Postfix expressions invoke the postfix operators on objects.
 
  \begin{grammar}
@@ skipping 1117 matching lines @@
 
 \commentary{
 In the simplest case, the immediately enclosing function is an ordinary, synchronous non-generator, and upon function termination, the current return value is given to the caller.  The other possibility is that the function is marked \ASYNC{}, in which case the current return value is used to complete the future associated with the function invocation. Both these scenarios are specified in section \ref{functionInvocation}.
 The enclosing function cannot be marked as generator (i.e, \ASYNC* or \SYNC*), since generators are not allowed to contain a statement of the form \code{\RETURN{} $e$;} as discussed below.
 }
 
 \LMHash{}
 Let $T$ be the static type of $e$ and let $f$ be the immediately enclosing function.  
 
 \LMHash{}
-It is a static type warning if the body of $f$ is marked \ASYNC{} and the type \code{Future$<$flatten(T)$>$} (\ref{awaitExpressions}) may not be assigned to the declared return type of $f$.   Otherwise, it is a static type warning if $T$ may not be assigned to the declared return type of $f$. 
+It is a static type warning if the body of $f$ is marked \ASYNC{} and the type \code{Future$<$flatten(T)$>$} (\ref{functionExpressions}) may not be assigned to the declared return type of $f$.    Otherwise, it is a static type warning if $T$ may not be assigned to the declared return type of $f$. 
 
 \LMHash{}
 Let $S$ be the runtime type of $o$. In checked mode:
 \begin{itemize}
 \item  If the body of $f$ is marked \ASYNC{} (\ref{functions}) it is a dynamic type error if $o$ is not \NULL{} (\ref{null}) and \code{Future$<$S$>$} is not a subtype of the actual return type  (\ref{actualTypeOfADeclaration}) of $f$.
 \item Otherwise, it is a dynamic type error if $o$ is not \NULL{} and the runtime type of $o$ is not a subtype of the actual return type of $f$.
 \end{itemize}
 
 \LMHash{}
 It is a compile-time error if a return statement of the form \code{\RETURN{} $e$;} appears in a generative constructor (\ref{generativeConstructors}).
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 after the associated \section{}, \subsection{} etc.
 
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