Add a function to define a style if not defined

modules/helper/README.md

@@ -13,6 +13,7 @@ Functions
   - `is-callable` checks if a name is a command, function, or alias.
   - `is-true` checks a boolean variable for "true".
   - `coalesce` prints the first non-empty string in the arguments array.
+  - `zstyle-` defines a style if it is already not defined.
 
 Authors
 -------

modules/helper/init.zsh

@@ -30,3 +30,7 @@ function coalesce {
   return 1
 }
 
+# Defines a style if it is already not defined.
+function zstyle- {
+  zstyle -T "$1" "$2" && zstyle "$@"
+}