code: public static <T> Mutable<T> fromExternal(final Supplier<T> s, final Consumer<T> c) {
          return new Mutable<T>() {
              @Override
              public T get() {
                  return s.get();
              }
  
              @Override
              public Mutable<T> set(final T value) {
                  c.accept(value);
                  return this;
              }
          };
      }
docs: Construct a Mutable that gets and sets an external value using the provided Supplier and Consumer
  
  e.g.
  <pre>
  {@code
  Mutable<Integer> mutable = Mutable.from(()->this.value*2,val->this.value=val);
  }
  </pre>
  
  
  @param s Supplier of an external value
  @param c Consumer that sets an external value
  @return Mutable that gets / sets an external (mutable) value