diff --git a/cylc/sphinx_ext/literal_sub_include/__init__.py b/cylc/sphinx_ext/literal_sub_include/__init__.py
index 459004a..4339885 100644
--- a/cylc/sphinx_ext/literal_sub_include/__init__.py
+++ b/cylc/sphinx_ext/literal_sub_include/__init__.py
@@ -59,8 +59,14 @@
 
 """
 
+from typing import TYPE_CHECKING
+
 from cylc.sphinx_ext.literal_sub_include.directives import LiteralSubInclude
 
+if TYPE_CHECKING:
+    from sphinx.application import Sphinx
+
+
 __version__ = '1.0.0'
 
 
diff --git a/cylc/sphinx_ext/literal_sub_include/directives.py b/cylc/sphinx_ext/literal_sub_include/directives.py
index 7a959fa..abfc104 100644
--- a/cylc/sphinx_ext/literal_sub_include/directives.py
+++ b/cylc/sphinx_ext/literal_sub_include/directives.py
@@ -17,23 +17,25 @@
 # -----------------------------------------------------------------------------
 
 from functools import wraps
+from typing import List
 
 from docutils.parsers.rst import directives
 from sphinx.directives.code import LiteralInclude, LiteralIncludeReader
 
 
-def substitute(lines, items):
+def substitute(lines: List[str], items: dict) -> None:
     """Substitute |strings| in the provided "lines" with values from "items".
 
     Example:
-        >>> lines = ['foo |bar| baz']
-        >>> substitute(lines, {'bar': 'pub'})
+        >>> lines = ['foo |bar| baz', 'foo bar |baz|']
+        >>> substitute(lines, {'bar': 'pub', 'baz': 'qux'})
         >>> lines
-        ['foo pub baz']
+        ['foo pub baz', 'foo bar qux']
     """
     for ind, line in enumerate(lines):
         for key, value in items.items():
-            lines[ind] = line.replace(f'|{key}|', value)
+            line = line.replace(f'|{key}|', value)
+        lines[ind] = line
 
 
 def substitution_decorator(fcn):