add explanation

cylc/flow/param_expand.py

@@ -70,6 +70,8 @@ def expand(template, params, results, values=None):
 REC_P_ALL = re.compile(r"(%s)?(?:<(.*?)>)?(.+)?" % TaskID.NAME_RE)
 # To extract all parameter lists e.g. 'm,n,o' (from '<m,n,o>').
 REC_P_GROUP = re.compile(r"<(.*?)>")
+# As REC_P_ALL, but retaining <> so that we can tell which bits of re.split
+# are templates, and which are plain text:
 REC_P_MATCH = re.compile(r'(<[^>]*>)')
 # To extract parameter name and optional offset or value e.g. 'm-1'.
 REC_P_OFFS = re.compile(
@@ -239,7 +241,6 @@ def _parse_task_name_string(parent):
             (['i = cat', 'j=3'], 'FAM{i}{j}')
         """
         tmpl_list = REC_P_MATCH.split(parent)
-
         param_list = []
         for template in tmpl_list:
             group = REC_P_GROUP.findall(template)