add emphasis for batch mode that the data is passed in params, not shared store.

docs/core_abstraction/batch.md

@@ -52,30 +52,75 @@ flow.run(shared)
 
 A **BatchFlow** runs a **Flow** multiple times, each time with different `params`. Think of it as a loop that replays the Flow for each parameter set.
 
+### Key Differences from BatchNode
+
+**Important**: Unlike BatchNode, which processes items and modifies the shared store:
+
+1. BatchFlow returns **parameters to pass to the child Flow**, not data to process
+2. These parameters are accessed in child nodes via `self.params`, not from the shared store
+3. Each child Flow runs independently with a different set of parameters
+4. Child nodes can be regular Nodes, not BatchNodes (the batching happens at the Flow level)
 
 ### Example: Summarize Many Files
 
 ```python
 class SummarizeAllFiles(BatchFlow):
     def prep(self, shared):
-        # Return a list of param dicts (one per file)
+        # IMPORTANT: Return a list of param dictionaries (not data for processing)
         filenames = list(shared["data"].keys())  # e.g., ["file1.txt", "file2.txt", ...]
         return [{"filename": fn} for fn in filenames]
 
-# Suppose we have a per-file Flow (e.g., load_file >> summarize >> reduce):
-summarize_file = SummarizeFile(start=load_file)
+# Child node that accesses filename from params, not shared store
+class LoadFile(Node):
+    def prep(self, shared):
+        # Access filename from params (not from shared)
+        filename = self.params["filename"]  # Important! Use self.params, not shared
+        return filename
         
-# Wrap that flow into a BatchFlow:
+    def exec(self, filename):
+        with open(filename, 'r') as f:
+            return f.read()
+            
+    def post(self, shared, prep_res, exec_res):
+        # Store file content in shared
+        shared["current_file_content"] = exec_res
+        return "default"
+
+# Summarize node that works on the currently loaded file
+class Summarize(Node):
+    def prep(self, shared):
+        return shared["current_file_content"]
+        
+    def exec(self, content):
+        prompt = f"Summarize this file in 50 words: {content}"
+        return call_llm(prompt)
+        
+    def post(self, shared, prep_res, exec_res):
+        # Store summary in shared, indexed by current filename
+        filename = self.params["filename"]  # Again, using params
+        if "summaries" not in shared:
+            shared["summaries"] = {}
+        shared["summaries"][filename] = exec_res
+        return "default"
+
+# Create a per-file flow
+load_file = LoadFile()
+summarize = Summarize()
+load_file >> summarize
+summarize_file = Flow(start=load_file)
+
+# Wrap in a BatchFlow to process all files
 summarize_all_files = SummarizeAllFiles(start=summarize_file)
 summarize_all_files.run(shared)
 ```
 
 ### Under the Hood
-1. `prep(shared)` returns a list of param dicts—e.g., `[{filename: "file1.txt"}, {filename: "file2.txt"}, ...]`.
+1. `prep(shared)` in the BatchFlow returns a list of param dicts—e.g., `[{"filename": "file1.txt"}, {"filename": "file2.txt"}, ...]`.
 2. The **BatchFlow** loops through each dict. For each one:
-   - It merges the dict with the BatchFlow’s own `params`.
-   - It calls `flow.run(shared)` using the merged result.
-3. This means the sub-Flow is run **repeatedly**, once for every param dict.
+   - It merges the dict with the BatchFlow's own `params` (if any): `{**batch_flow.params, **dict_from_prep}`
+   - It calls `flow.run(shared)` using the merged parameters
+   - **IMPORTANT**: These parameters are passed to the child Flow's nodes via `self.params`, NOT via the shared store
+3. This means the sub-Flow is run **repeatedly**, once for every param dict, with each node in the flow accessing the parameters via `self.params`.
 
 ---
 
@@ -91,6 +136,7 @@ At each level, **BatchFlow** merges its own param dict with the parent’s. By t
 
 class FileBatchFlow(BatchFlow):
     def prep(self, shared):
+        # Access directory from params (set by parent)
         directory = self.params["directory"]
         # e.g., files = ["file1.txt", "file2.txt", ...]
         files = [f for f in os.listdir(directory) if f.endswith(".txt")]
@@ -101,7 +147,31 @@ class DirectoryBatchFlow(BatchFlow):
         directories = [ "/path/to/dirA", "/path/to/dirB"]
         return [{"directory": d} for d in directories]
 
-# MapSummaries have params like {"directory": "/path/to/dirA", "filename": "file1.txt"}
-inner_flow = FileBatchFlow(start=MapSummaries())
+# The actual processing node
+class ProcessFile(Node):
+    def prep(self, shared):
+        # Access both directory and filename from params
+        directory = self.params["directory"]  # From outer batch
+        filename = self.params["filename"]    # From inner batch
+        full_path = os.path.join(directory, filename)
+        return full_path
+        
+    def exec(self, full_path):
+        # Process the file...
+        return f"Processed {full_path}"
+        
+    def post(self, shared, prep_res, exec_res):
+        # Store results, perhaps indexed by path
+        if "results" not in shared:
+            shared["results"] = {}
+        shared["results"][prep_res] = exec_res
+        return "default"
+
+# Set up the nested batch structure
+process_node = ProcessFile()
+inner_flow = FileBatchFlow(start=process_node)
 outer_flow = DirectoryBatchFlow(start=inner_flow)
+
+# Run it
+outer_flow.run(shared)
 ```