fixup: add 34.9.9 finished

Author: jensmaurer

source/exec.tex

@@ -2536,9 +2536,42 @@
 
 \rSec3[exec.read.env]{\tcode{execution::read_env}}
 
+\pnum
+\tcode{read_env} is a sender factory for a sender
+whose asynchronous operation completes synchronously in its start operation
+with a value completion result equal to
+a value read from the receiver's associated environment.
+
+\pnum
+\tcode{read_env} is a customization point object.
+For some query object \tcode{q},
+the expression \tcode{read_env(q)} is expression-equivalent to
+\tcode{\exposid{make-sender}(read_env, q)}.
+
+\pnum
+The exposition-only class template \exposid{impls-for}\iref{exec.snd.general}
+is specialized for \tcode{read_env} as follows:
+\begin{codeblock}
+namespace std::execution {
+  template<>
+  struct @\exposid{impls-for}@<@\exposid{decayed-typeof}@<read_env>> : @\exposid{default-impls}@ {
+    static constexpr auto start =
+      [](auto query, auto& rcvr) noexcept -> void {
+        @\exposid{TRY-SET-VALUE}@(rcvr, query(get_env(rcvr)));
+      };
+  };
+}
+\end{codeblock}
+
+\rSec2[exec.adapt]{Sender adaptors}
+
+\rSec3[exec.adapt.general]{General}
+
+
 
 \rSec1[exec.util]{Sender/receiver utilities}
 
+
 \rSec1[exec.ctx]{Execution contexts}
 
 \rSec1[exec.coro.util]{Coroutine utilities}