Rename main class from $ to U.

Author: javadev

…n/java/com/github/underscore/math/$.java …n/java/com/github/underscore/math/U.javamath-plugin/src/main/java/com/github/underscore/math/$.java renamed to math-plugin/src/main/java/com/github/underscore/math/U.java math-plugin/src/main/java/com/github/underscore/math/$.java renamed to math-plugin/src/main/java/com/github/underscore/math/U.java

@@ -54,8 +54,8 @@ public boolean isPresent() {
         return !absent;
     }
 
-    public <U> Optional<? extends U> map(Function<? super T, ? extends U> mapper) {
-        $.checkNotNull(mapper);
+    public <F> Optional<? extends F> map(Function<? super T, ? extends F> mapper) {
+        U.checkNotNull(mapper);
         if (!isPresent()) {
             return absent();
         } else {