diff --git a/src/xul/cmd/xp.py b/src/xul/cmd/xp.py
index dd8d0b3..10ee39f 100644
--- a/src/xul/cmd/xp.py
+++ b/src/xul/cmd/xp.py
@@ -102,7 +102,8 @@ def xpath_class(el_tree: etree._ElementTree, xpath_exp: str, ns_map: dict[str, s
 
     :param el_tree: lxml ElementTree
     :param xpath_exp: XPath expression
-    :param ns_map: XML namespaces (xmlns) 'prefix': 'URI' dict
+    :param ns_map: XML namespace (prefix: URI) dictionary
+    :return: XPath result
     """
     if xpath_obj := build_xpath(xpath_exp, ns_map):
         return etree_xpath(el_tree, xpath_obj)
@@ -114,16 +115,17 @@ def eltree_xpath(el_tree: etree._ElementTree, xpath_exp: str, ns_map: dict[str,
 
     :param el_tree: lxml ElementTree
     :param xpath_exp: XPath expression
-    :param ns_map: XML namespaces (xmlns) 'prefix': 'URI' dict
+    :param ns_map: XML namespace (prefix: URI) dictionary
+    :return: XPath result
     """
     try:
         return el_tree.xpath(xpath_exp, namespaces=ns_map)
     except etree.XPathEvalError as e:
         sys.stderr.write(f"{e}: {xpath_exp}\n")
         return None
-    # EXSLT function call errors (re:test positional arguments).
+    # Incorrect EXSLT function call (e.g. number of positional arguments for re:test).
     except TypeError as e:
-        sys.stderr.write(f"Type error {e}: {xpath_exp}\n")
+        sys.stderr.write(f"{xpath_exp} is invalid: {e}\n")
         return None
 
 
@@ -153,7 +155,7 @@ def xp_prepare(
 def print_xmlns(ns_map: dict[str, str], root: etree._Element) -> None:
     """Print XML source namespaces (prefix: namespace URI).
 
-    :param ns_map: XML namespaces (xmlns) 'prefix': 'URI' dict
+    :param ns_map: XML namespace (prefix: URI) dictionary
     :param root: root (document) element
     """
     if ns_map:
@@ -322,12 +324,6 @@ def print_result_list(result_list, el_tree: etree._ElementTree, args: argparse.N
             else:
                 print(f"prefix: {prefix:<8} URI: {uri}")
 
-        # ?
-        else:
-            print("**DEBUG fallback**")
-            print(type(node))
-            print(node)
-
 
 def print_result_header(source_name: str, xp_result) -> None:
     """Print header with XPath result summary.
diff --git a/src/xul/xpath.py b/src/xul/xpath.py
index 6b430f2..e2d9bb3 100644
--- a/src/xul/xpath.py
+++ b/src/xul/xpath.py
@@ -1,9 +1,13 @@
 """XPath.
 
-XPath with lxml
+XPath:
     https://lxml.de/xpathxslt.html#xpath
 
-The XPath result depends on the XPath expression used.
+The XPath class:
+    https://lxml.de/xpathxslt.html#the-xpath-class
+    https://lxml.de/apidoc/lxml.etree.html#lxml.etree.XPath
+
+The return value types of XPath evaluations vary, depending on the XPath expression used:
     https://lxml.de/xpathxslt.html#xpath-return-values
 - attributes:               "location/@attribute"
 - text nodes:               "location/text()"
@@ -16,62 +20,58 @@
 """
 
 from logging import getLogger
+from typing import Optional, TextIO, Union
 
-# pylint: disable=no-name-in-module
-from lxml.etree import LIBXSLT_COMPILED_VERSION, XPath, XPathEvalError, XPathSyntaxError
+from lxml import etree
 
-# Import my own modules.
 from .etree import build_etree
 
-# Module logging initialisation.
 logger = getLogger(__name__)
 
 
-def build_xpath(xpath_exp, ns_map=None):
+def build_xpath(xpath_exp: str, ns_map: Optional[dict[str, str]] = None) -> Optional[etree.XPath]:
     """Build an lxml.etree.XPath instance from an XPath expression.
 
-    xpath_exp -- XPath expression
-    ns_map -- XML namespace (prefix: URI) dictionary
-
-    Uses the lxml XPath class:
-        https://lxml.de/xpathxslt.html#the-xpath-class
-        https://lxml.de/apidoc/lxml.etree.html#lxml.etree.XPath
+    :param xpath_exp: XPath expression
+    :param ns_map: XML namespace (prefix: URI) dictionary
     """
     if not ns_map:
         ns_map = {}
     try:
-        return XPath(xpath_exp, namespaces=ns_map)
+        return etree.XPath(xpath_exp, namespaces=ns_map)
     # Handle (parsing) errors in XPath expression.
     #   https://lxml.de/xpathxslt.html#error-handling
-    except XPathSyntaxError as e:
+    except etree.XPathSyntaxError as e:
         logger.error("%s: %s", e, xpath_exp)
         return None
 
 
-def etree_xpath(el_tree, xpath_obj):
+def etree_xpath(el_tree: etree._ElementTree, xpath_obj: etree.XPath):
     """Apply XPath instance to an ElementTree.
 
-    el_tree -- ElementTree (lxml.etree._ElementTree)
-    xpath_obj -- lxml.etree.XPath instance; see build_xpath()
+    :param el_tree: lxml ElementTree
+    :param xpath_obj: lxml.etree.XPath instance; see build_xpath()
+    :return: XPath result
     """
     try:
         return xpath_obj(el_tree)
     # Handle errors in evaluating an XPath expression.
     #   https://lxml.de/xpathxslt.html#error-handling
-    except XPathEvalError as e:
+    except etree.XPathEvalError as e:
         logger.error("%s: %s", e, xpath_obj)
         return None
-    # Incorrect EXSLT function call (e.g. number of arguments for re:match).
+    # Incorrect EXSLT function call (e.g. number of positional arguments for re:match).
     except TypeError as e:
-        logger.error("Type error %s: %s", e, xpath_obj)
+        logger.error("%s is invalid: %s", xpath_obj, e)
         return None
 
 
-def call_xpath(xml_source, xpath_obj):
-    """Apply lxml.etree.XPath on an XML source.
+def call_xpath(xml_source: Union[TextIO, str], xpath_obj: etree.XPath):
+    """Apply lxml.etree.XPath to an XML source.
 
-    xml_source -- XML file, file-like object or URL
-    xpath_obj -- lxml.etree.XPath instance; see build_xpath()
+    :param xml_source: XML file, file-like object or URL
+    :param xpath_obj: lxml.etree.XPath instance; see build_xpath()
+    :return: XPath result
     """
     # Parse an XML source into an XML Document Object Model.
     el_tree = build_etree(xml_source, lenient=False)
@@ -85,27 +85,27 @@ def call_xpath(xml_source, xpath_obj):
     return xpath_result
 
 
-def xml_xpath(xml_source, xpath_exp):
-    """Apply XPath expression to an XML source.
+def xml_xpath(xml_source: Union[TextIO, str], xpath_exp: str):
+    """Apply XPath expression to an XML source with call_xpath().
 
-    xml_source -- XML file, file-like object or URL
-    xpath_exp -- XPath expression
-
-    Uses call_xpath()
+    :param xml_source: XML file, file-like object or URL
+    :param xpath_exp: XPath expression
+    :return: XPath result
     """
-    xpath_obj = build_xpath(xpath_exp)
-    if not xpath_obj:
-        return None
+    if xpath_obj := build_xpath(xpath_exp):
+        return call_xpath(xml_source, xpath_obj)
 
-    return call_xpath(xml_source, xpath_obj)
+    return None
 
 
-def update_ns_map(ns_map, elm, none_prefix="default"):
+def update_ns_map(
+    ns_map: dict[str, str], elm: etree._Element, none_prefix: str = "default"
+) -> None:
     """Update XPath namespace prefix mapping with element namespaces.
 
-    ns_map -- an XML namespace prefix mapping
-    elm -- element with namespaces
-    none_prefix -- prefix for the default namespace in XPath
+    :param ns_map: XML namespace (prefix: URI) dictionary
+    :param elm: element with namespaces
+    :param none_prefix: prefix for the default namespace in XPath
 
     Element namespaces:
     - xmlns, default namespace (None prefix) URI: elm.nsmap[None]
@@ -121,35 +121,36 @@ def update_ns_map(ns_map, elm, none_prefix="default"):
         https://lxml.de/xpathxslt.html#namespaces-and-prefixes
     """
     for key in elm.nsmap:
-        if not key:
+        if key is None:
             # XPath prefix for element default namespace.
             if none_prefix not in ns_map:
                 ns_map[none_prefix] = elm.nsmap[key]
-        elif key not in ns_map:
-            # Protect the XPath default namespace prefix.
-            if not key == none_prefix:
-                ns_map[key] = elm.nsmap[key]
+        # Protect the XPath default namespace prefix.
+        elif not (key in ns_map or key == none_prefix):
+            ns_map[key] = elm.nsmap[key]
 
 
-def namespaces(el_tree, exslt=False, none_prefix="default"):
+def namespaces(
+    el_tree: etree._ElementTree, exslt: bool = False, none_prefix: str = "default"
+) -> dict[str, str]:
     """Collect all XML namespaces (xmlns) in ElementTree.
 
-    el_tree -- ElementTree (lxml.etree._ElementTree)
-    exslt -- add EXSLT XML namespace prefixes (libxslt 1.1.25 and newer)
-    none_prefix -- prefix for the default namespace in XPath
+    :param el_tree: lxml ElementTree
+    :param exslt: add EXSLT XML namespace prefixes (libxslt 1.1.25 and newer)
+    :param none_prefix: prefix for the default namespace in XPath
 
-    Return XML namespaces (xmlns) 'prefix: URI' dict.
+    Return XML namespaces (xmlns) 'prefix: URI' mapping.
 
     Namespaces.
         https://lxml.de/tutorial.html#namespaces
     """
     if exslt:
-        if LIBXSLT_COMPILED_VERSION < (1, 1, 25):
+        if etree.LIBXSLT_COMPILED_VERSION < (1, 1, 25):
             logger.warning(
-                "EXSLT requires libxslt 1.1.25 or higher. " + "lxml is compiled against libxslt %s",
-                ".".join(str(n) for n in LIBXSLT_COMPILED_VERSION),
+                "EXSLT requires libxslt 1.1.25 or higher. lxml is compiled against libxslt %s",
+                ".".join(str(n) for n in etree.LIBXSLT_COMPILED_VERSION),
             )
-        # EXSLT <http://exslt.org/>
+        # EXSLT <https://exslt.github.io/>
         ns_map = {
             "date": "http://exslt.org/dates-and-times",
             "dyn": "http://exslt.org/dynamic",