diff --git a/runtime/doc/if_pyth.txt b/runtime/doc/if_pyth.txt
index d21de4b..a93f3cb 100644
--- a/runtime/doc/if_pyth.txt
+++ b/runtime/doc/if_pyth.txt
@@ -82,6 +82,18 @@
 	:pydo return "%s\t%d" % (line[::-1], len(line))
 	:pydo if line: return "%4d: %s" % (linenr, line)
 <
+One can use `:pydo` in possible conjunction with `:py` to filter a range using
+python. For example: >
+
+	:py3 << EOF
+	needle = vim.eval('@a')
+	replacement = vim.eval('@b')
+
+	def py_vim_string_replace(str):
+		return str.replace(needle, replacement)
+	EOF
+	:'<,'>py3do return py_vim_string_replace(line)
+<
 							*:pyfile* *:pyf*
 :[range]pyf[ile] {file}
 			Execute the Python script in {file}.  The whole