diff --git a/src/policies/agents/agent.jl b/src/policies/agents/agent.jl
index 4d2cfde..817c6c9 100644
--- a/src/policies/agents/agent.jl
+++ b/src/policies/agents/agent.jl
@@ -31,10 +31,28 @@ function check(agent::Agent, env::AbstractEnv)
     check(agent.policy, env)
 end
 
-#####
-# update!
-#####
-
+"""
+Here we extend the definition of `(p::AbstractPolicy)(::AbstractEnv)` in
+`RLBase` to accept an `AbstractStage` as the first argument. Algorithm designers
+may customize these behaviors respectively. The default behaviors are:
+1. Update the inner `trajectory` given the context of `policy`, `env`, and
+   `stage`.
+  1. By default we do nothing.
+  2. In `PreActStage`, we `push!` the current **state** of the `env` and the
+     **action** generated by `policy(env)` into the `trajectory`. And the
+     **action** is returned.
+  3. In `PostActStage`, we query the `reward` and `is_terminated` info from
+     `env` and push them into `trajectory`.
+  4. For `CircularSARTTrajectory`:
+     1. In the `PosEpisodeStage`, we push the `state` at the end of an episode
+        and a dummy action into the `trajectory`.
+     1. In the `PreEpisodeStage`, we pop out the lastest `state` and `action`
+        pair (which are dummy ones) from `trajectory`.
+2. Update the inner `policy` given the context of `trajectory`, `env`, and
+   `stage`.
+  1. By default, we only `update!` the `policy` in the `PreActStage`. And it's
+     despatched to `update!(policy, trajectory)`.
+"""
 function (agent::Agent)(stage::AbstractStage, env::AbstractEnv)
     update!(agent.trajectory, agent.policy, env, stage)
     update!(agent.policy, agent.trajectory, env, stage)
@@ -48,11 +66,10 @@ end
 
 RLBase.update!(::AbstractPolicy, ::AbstractTrajectory, ::AbstractEnv, ::AbstractStage) =
     nothing
+
 RLBase.update!(p::AbstractPolicy, t::AbstractTrajectory, ::AbstractEnv, ::PreActStage) =
     update!(p, t)
 
-## update trajectory
-
 RLBase.update!(::AbstractTrajectory, ::AbstractPolicy, ::AbstractEnv, ::AbstractStage) =
     nothing